docs(README): fix missing word (graphql#595)

README.md

@@ -99,7 +99,7 @@ The `graphqlHTTP` function accepts the following options:
 - **`pretty`**: If `true`, any JSON response will be pretty-printed.
 
 - **`extensions`**: An optional function for adding additional metadata to the
-  GraphQL response as a key-value object. The result will be added to
+  GraphQL response as a key-value object. The result will be added to the
   `"extensions"` field in the resulting JSON. This is often a useful place to
   add development time metadata such as the runtime of a query or the amount
   of resources consumed. This may be an async function. The function is