minor #13781 Fix typo in the markup (fabpot)

This PR was merged into the 3.4 branch.

Discussion
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Fix typo in the markup

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Commits
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cf9042a Fix typo in the markup

Author: javiereguiluz

contributing/code/bc.rst

@@ -32,7 +32,7 @@ The second section, "Working on Symfony Code", is targeted at Symfony
 contributors. This section lists detailed rules that every contributor needs to
 follow to ensure smooth upgrades for our users.
 
-.. warning::
+.. caution::
 
     :doc:`Experimental Features </contributing/code/experimental>` and code
     marked with the ``@internal`` tags are excluded from our Backward