fixed error in ContentDisposition doc (#7154)

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## What do these changes do?
Fix small mistake in doc

## Are there changes in behavior for the user?
No.

## Related issue number

#7151

Co-authored-by: Solar Joe <solarjoe@posteo.org>

docs/client_reference.rst

@@ -2085,7 +2085,7 @@ All exceptions are available as members of *aiohttp* module.
 
     Represent Content-Disposition header
 
-    .. attribute:: value
+    .. attribute:: type
 
     A :class:`str` instance. Value of Content-Disposition header
     itself, e.g. ``attachment``.