[PATCH v1] Fix typos: "its a" -> "it's a" in code comments

Hi hackers,

Attached is a small patch that fixes two instances where the possessive "its" was incorrectly used instead of the contraction "it's" (it is) in code comments.

Affected files:
- src/backend/executor/execTuples.c
- contrib/btree_gist/btree_utils_var.h

The patch applies cleanly against current HEAD (dd5716f3c74) and passes git diff --check with no whitespace issues.

Best regards,
Thiago Caserta <https://aka.ms/GetOutlookForMac&gt;