Correct grammar/typo in runtime instance.rs (#628)

crates/runtime/src/instance.rs

@@ -881,7 +881,7 @@ impl InstanceHandle {
 
     /// Return an iterator over the exports of this instance.
     ///
-    /// Specifically, it provides access to the key-value pairs, where they keys
+    /// Specifically, it provides access to the key-value pairs, where the keys
     /// are export names, and the values are export declarations which can be
     /// resolved `lookup_by_declaration`.
     pub fn exports(&self) -> indexmap::map::Iter<String, wasmtime_environ::Export> {