minor #21186 Fix session lifetime index name in examples (whataboutpereira)

This PR was submitted for the 7.3 branch but it was merged into the 6.4 branch instead.

Discussion
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Fix session lifetime index name in examples

Session index is generated using the lifetime column name.

symfony/symfony@1a67728#diff-fb1dcfa62129a037c11aebcf43264282a5c8e2da5d9409aa8f4f3281659b3dd1R263

However current the examples are prepending 'sessions_' to the generated index name.

This results in a migration being created to rename `sessions_sess_lifetime_idx` to `sess_lifetime_idx` after creating the table using the SQL in the docs.

Commits
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6d0f066 Fix session lifetime index name in examples

Author: javiereguiluz

session.rst

@@ -966,7 +966,7 @@ MariaDB/MySQL
         `sess_data` BLOB NOT NULL,
         `sess_lifetime` INTEGER UNSIGNED NOT NULL,
         `sess_time` INTEGER UNSIGNED NOT NULL,
-        INDEX `sessions_sess_lifetime_idx` (`sess_lifetime`)
+        INDEX `sess_lifetime_idx` (`sess_lifetime`)
     ) COLLATE utf8mb4_bin, ENGINE = InnoDB;
 
 .. note::
@@ -987,7 +987,7 @@ PostgreSQL
         sess_lifetime INTEGER NOT NULL,
         sess_time INTEGER NOT NULL
     );
-    CREATE INDEX sessions_sess_lifetime_idx ON sessions (sess_lifetime);
+    CREATE INDEX sess_lifetime_idx ON sessions (sess_lifetime);
 
 Microsoft SQL Server
 ++++++++++++++++++++
@@ -999,7 +999,7 @@ Microsoft SQL Server
         sess_data NVARCHAR(MAX) NOT NULL,
         sess_lifetime INTEGER NOT NULL,
         sess_time INTEGER NOT NULL,
-        INDEX sessions_sess_lifetime_idx (sess_lifetime)
+        INDEX sess_lifetime_idx (sess_lifetime)
     );
 
 .. _session-database-mongodb: