Use aiohttp namespace for example in client_quickstart.rst (#7269)

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## What do these changes do?

Formdata() was modified to aiohttp.Formdata() because import was not
done separately. This kind of modification is necessary because an error
occurs if you execute only the example code.

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## Are there changes in behavior for the user?

No. just docs.

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- [x] Unit tests for the changes exist
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docs/client_quickstart.rst

@@ -316,7 +316,7 @@ To upload Multipart-encoded files::
 You can set the ``filename`` and ``content_type`` explicitly::
 
     url = 'http://httpbin.org/post'
-    data = FormData()
+    data = aiohttp.FormData()
     data.add_field('file',
                    open('report.xls', 'rb'),
                    filename='report.xls',