minor #19726 Update session.rst (i07)

This PR was submitted for the 7.0 branch but it was merged into the 5.4 branch instead.

Discussion
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Update session.rst

using Redis as class is in error, the class needs to be specified as \Redis as that is the correct namespace.

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Commits
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154efb5 Update session.rst

Author: javiereguiluz

session.rst

@@ -541,7 +541,7 @@ a Symfony service for the connection to the Redis server:
 
             Redis:
                 # you can also use \RedisArray, \RedisCluster or \Predis\Client classes
-                class: Redis
+                class: \Redis
                 calls:
                     - connect:
                         - '%env(REDIS_HOST)%'