minor #16719 Replace duplicate medium configuration (ncsmnicole)

This PR was submitted for the 6.0 branch but it was merged into the 5.4 branch instead.

Discussion
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Replace duplicate medium configuration

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Commits
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3feb2ba Replace duplicate medium configuration

Author: javiereguiluz

notifier.rst

@@ -601,7 +601,7 @@ specify what channels should be used for specific levels (using
                 ->channelPolicy('high', ['chat/slack'])
                 // Use browser for medium and low notifications
                 ->channelPolicy('medium', ['browser'])
-                ->channelPolicy('medium', ['browser'])
+                ->channelPolicy('low', ['browser'])
             ;
         };