Replace utf8_bin collation with utf8mb4_bin in example

Author: bocharsky-bw

doctrine/pdo_session_storage.rst

@@ -255,7 +255,7 @@ MySQL
         `sess_data` BLOB NOT NULL,
         `sess_time` INTEGER UNSIGNED NOT NULL,
         `sess_lifetime` MEDIUMINT NOT NULL
-    ) COLLATE utf8_bin, ENGINE = InnoDB;
+    ) COLLATE utf8mb4_bin, ENGINE = InnoDB;
 
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