minor #17075 [Mercure] Update mercure.rst (ousmaneNdiaye)

This PR was submitted for the 6.1 branch but it was merged into the 4.4 branch instead.

Discussion
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[Mercure] Update mercure.rst

Fix typo

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Commits
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d2f676e Update mercure.rst

Author: javiereguiluz

mercure.rst

@@ -109,7 +109,7 @@ to the Mercure Hub to be authorized to publish updates and, sometimes, to subscr
 This JWT should be stored in the ``MERCURE_JWT_SECRET`` environment variable.
 
 The JWT must be signed with the same secret key as the one used by
-the Hub to verify the JWT (``!ChangeMe!`` in you use the Local Web Server or
+the Hub to verify the JWT (``!ChangeMe!`` if you use the Local Web Server or
 Symfony Docker).
 Its payload must contain at least the following structure to be allowed to
 publish: