Update mg_file_upload_handler.md

docs/c-api/http.h/mg_file_upload_handler.md

@@ -12,7 +12,7 @@ This handler can be used to implement file uploads with minimum code.
 This handler will process MG_EV_HTTP_PART_* events and store file data into
 a local file.
 `local_name_fn` will be invoked with whatever name was provided by the client
-and will expect the name of the local file to open. Return value of NULL will
+and will expect the name of the local file to open. A return value of NULL will
 abort file upload (client will get a "403 Forbidden" response). If non-null,
 the returned string must be heap-allocated and will be freed by the caller.
 Exception: it is ok to return the same string verbatim.