[3.12] Fix variable name in dis documentation example (GH-109343) (#109364)

Fix variable name in dis documentation example (GH-109343)

BINARY_SUBSCR example erroneously uses two different names `key` and `index` to refer to the same variable. STORE_SUBSCR and DELETE_SUBSCR use only `key` in the same context. Changing `index` to `key` for consistency.
(cherry picked from commit a0c06a4f933faccd7f8201701b2491d38464212c)

Co-authored-by: Oleksandr Kravets <73752159+olekskrav@users.noreply.github.com>