Change 'Cluster' to 'Cartridge' in the main menu (#2744)

Resolves #1996

According to two native speakers, "Cluster on Cartridge" sounds fine.

Author: patiencedaur

doc/toctree.rst

@@ -13,7 +13,7 @@
     book/box/authentication
     book/box/triggers
     reference/reference_rock/vshard/vshard_index
-    Cluster <book/cartridge/index>
+    Cluster on Cartridge <book/cartridge/index>
     book/app_server/index
     book/admin/index
     book/replication/index