Fix comment in procarray.c.

Commit fea10a6 renamed VariableCacheData.nextFullXid to nextXid.
But commit dc7420c introduced the comment mentioning nextFullXid.
This commit changes"nextFullXid" to "nextXid" in the comment.

Author: Zhang Mingli
Discussion: https://postgr.es/m/642BA615-4B28-4B0C-BDF6-4D33E366BCDF@gmail.com

Author: MasaoFujii

src/backend/storage/ipc/procarray.c

@@ -4212,8 +4212,8 @@ GlobalVisTestIsRemovableXid(GlobalVisState *state, TransactionId xid)
 	/*
 	 * Convert 32 bit argument to FullTransactionId. We can do so safely
 	 * because we know the xid has to, at the very least, be between
-	 * [oldestXid, nextFullXid), i.e. within 2 billion of xid. To avoid taking
-	 * a lock to determine either, we can just compare with
+	 * [oldestXid, nextXid), i.e. within 2 billion of xid. To avoid taking a
+	 * lock to determine either, we can just compare with
 	 * state->definitely_needed, which was based on those value at the time
 	 * the current snapshot was built.
 	 */