Fix typo

Noticed while reviewing code.

src/backend/commands/vacuumlazy.c

@@ -529,7 +529,7 @@ lazy_scan_heap(Relation onerel, int options, LVRelStats *vacrelstats,
 	 * safely set for relfrozenxid or relminmxid.
 	 *
 	 * Before entering the main loop, establish the invariant that
-	 * next_unskippable_block is the next block number >= blkno that's not we
+	 * next_unskippable_block is the next block number >= blkno that we
 	 * can't skip based on the visibility map, either all-visible for a
 	 * regular scan or all-frozen for an aggressive scan.  We set it to
 	 * nblocks if there's no such block.  We also set up the skipping_blocks