minor #12607 Update mercure.rst (gponty)

This PR was merged into the 4.3 branch.

Discussion
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Update mercure.rst

Just replace MERCURE_JWT_SECRET by MERCURE_JWT_TOKEN.

symfony/recipes@9a58252#diff-d19ccce8daab2b00dbfa4e827ebde212

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Commits
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cd74623 Update mercure.rst

Author: wouterj

mercure.rst

@@ -99,7 +99,7 @@ Set it to the URL of the Mercure Hub (``http://localhost:3000/.well-known/mercur
 In addition, the Symfony application must bear a `JSON Web Token`_ (JWT)
 to the Mercure Hub to be authorized to publish updates.
 
-This JWT should be stored in the ``MERCURE_JWT_SECRET`` environment variable.
+This JWT should be stored in the ``MERCURE_JWT_TOKEN`` environment variable.
 
 The JWT must be signed with the same secret key as the one used by
 the Hub to verify the JWT (``aVerySecretKey`` in our example).
@@ -126,7 +126,7 @@ public updates (see the authorization_ section for further information).
 
 .. caution::
 
-    Don't put the secret key in ``MERCURE_JWT_SECRET``, it will not work!
+    Don't put the secret key in ``MERCURE_JWT_TOKEN``, it will not work!
     This environment variable must contain a JWT, signed with the secret key.
 
     Also, be sure to keep both the secret key and the JWTs... secrets!