minor #8356 Reworded the prepare() method explanation (javiereguiluz)

This PR was merged into the 2.7 branch.

Discussion
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Reworded the prepare() method explanation

After reading #8348, I think we could improve `prepare()` method explanation a bit. If you don't agree, please close because this is a very minor change. Thanks!

Commits
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674b7b4 Reworded the prepare() method explanation

Author: xabbuh

components/http_foundation.rst

@@ -356,9 +356,9 @@ UTF-8.
 Sending the Response
 ~~~~~~~~~~~~~~~~~~~~
 
-Before sending the Response, you can ensure that it is compliant with the HTTP
-specification by calling the
-:method:`Symfony\\Component\\HttpFoundation\\Response::prepare` method::
+Before sending the Response, you can optionally call the
+:method:`Symfony\\Component\\HttpFoundation\\Response::prepare` method to fix any
+incompatibility with the HTTP specification (e.g. a wrong ``Content-Type`` header)::
 
     $response->prepare($request);