Update README.md

Author: samuelhwilliams

README.md

@@ -252,7 +252,7 @@ the JavaScript side, which defaults to 10000 milliseconds (10 seconds). This can
 
 #### Callbacks
 
-When you call an exposed function, you can immediately pass a callback function afterwards. This callback will automatically be called asynchronous with the return value when the function has finished executing on the other side.
+When you call an exposed function, you can immediately pass a callback function afterwards. This callback will automatically be called asynchronously with the return value when the function has finished executing on the other side.
 
 For example, if we have the following function defined and exposed in Javascript: