Fix typo (#553)

docs/WASI-tutorial.md

@@ -288,7 +288,7 @@ First, create a new `demo.wat` file:
             (i32.const 1) ;; file_descriptor - 1 for stdout
             (i32.const 0) ;; *iovs - The pointer to the iov array, which is stored at memory location 0
             (i32.const 1) ;; iovs_len - We're printing 1 string stored in an iov - so one.
-            (i32.const 20) ;; nwritten - A place in memory to store the number of bytes writen
+            (i32.const 20) ;; nwritten - A place in memory to store the number of bytes written
         )
         drop ;; Discard the number of bytes written from the top the stack
     )