fix error in doc (#35)

* fix error in doc

README.md

@@ -129,7 +129,7 @@ mockedHTTPClient := mock.NewMockedHTTPClient(
     mock.WithRequestMatchHandler(
         mock.GetUsersByUsername,
         http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
-            WriteError(
+            mock.WriteError(
                 w,
                 http.StatusInternalServerError,
                 "github went belly up or something",