Fix spelling of MP_QSTR

MP_QSTR was mispelled as MP_QSTR in a few places.

docs/micropython-usermod.ipynb

@@ -851,7 +851,7 @@
    "source": [
     "At this point, we are more or less done with the C implementation of our function, but we still have to expose it. This we do by adding a table, an array of key/value pairs to the globals of our module, and bind the table to the `_module_globals` variable by applying the `MP_DEFINE_CONST_DICT` macro. This table should have at least one entry, the name of the module, which is going to be stored in the string `MP_QSTR___name__`. \n",
     "\n",
-    "These `MP_QSRT_` items are the C representation of the python strings that come at the end of them. So, `MP_QSRT_foo_bar` in C will be turned into a name, `foo_bar`, in python. `foo_bar` can be a constant, a function, a class, a type, etc., and depending on what is associated with it, different things will happen on the console, when `foo_bar` is invoked. But the crucial point is that, if you want `foo_bar` to have any meaning in python, then somewhere in your C code, you have to define `MP_QSRT_foo_bar`. \n",
+    "These `MP_QSTR_` items are the C representation of the python strings that come at the end of them. So, `MP_QSTR_foo_bar` in C will be turned into a name, `foo_bar`, in python. `foo_bar` can be a constant, a function, a class, a type, etc., and depending on what is associated with it, different things will happen on the console, when `foo_bar` is invoked. But the crucial point is that, if you want `foo_bar` to have any meaning in python, then somewhere in your C code, you have to define `MP_QSTR_foo_bar`. \n",
     "\n",
     "The second key-value pair of the table is the pointer to the function that we have just implemented, and the name that we want to call the functions in python itself. So, in the example below, our `simplefunction_add_ints` function will be invoked, when we call `add_ints` in the console.\n",
     "\n",

docs/source/usermods_05.rst

@@ -119,14 +119,14 @@ table to the ``_module_globals`` variable by applying the
 entry, the name of the module, which is going to be stored in the string
 ``MP_QSTR___name__``.
 
-These ``MP_QSRT_`` items are the C representation of the python strings
+These ``MP_QSTR_`` items are the C representation of the python strings
-that come at the end of them. So, ``MP_QSRT_foo_bar`` in C will be
+that come at the end of them. So, ``MP_QSTR_foo_bar`` in C will be
 turned into a name, ``foo_bar``, in python. ``foo_bar`` can be a
 constant, a function, a class, a type, etc., and depending on what is
 associated with it, different things will happen on the console, when
 ``foo_bar`` is invoked. But the crucial point is that, if you want
 ``foo_bar`` to have any meaning in python, then somewhere in your C
-code, you have to define ``MP_QSRT_foo_bar``.
+code, you have to define ``MP_QSTR_foo_bar``.
 
 The second key-value pair of the table is the pointer to the function
 that we have just implemented, and the name that we want to call the