Fix session lifetime index name in examples

Session index is generated using the lifetime column name, 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.

Author: whataboutpereira

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: