Fix grammar mistakes in md.c

Author: Kirk Jamison
Discussion: https://postgr.es/m/D09B13F772D2274BB348A310EE3027C640AC54@g01jpexmbkw24

src/backend/storage/smgr/md.c

@@ -171,7 +171,7 @@ static CycleCtr mdckpt_cycle_ctr = 0;
 #define EXTENSION_CREATE_RECOVERY	(1 << 3)
 /*
  * Allow opening segments which are preceded by segments smaller than
- * RELSEG_SIZE, e.g. inactive segments (see above). Note that this is breaks
+ * RELSEG_SIZE, e.g. inactive segments (see above). Note that this breaks
  * mdnblocks() and related functionality henceforth - which currently is ok,
  * because this is only required in the checkpointer which never uses
  * mdnblocks().
@@ -882,8 +882,8 @@ mdnblocks(SMgrRelation reln, ForkNumber forknum)
 		segno++;
 
 		/*
-		 * We used to pass O_CREAT here, but that's has the disadvantage that
-		 * it might create a segment which has vanished through some operating
+		 * We used to pass O_CREAT here, but that has the disadvantage that it
+		 * might create a segment which has vanished through some operating
 		 * system misadventure.  In such a case, creating the segment here
 		 * undermines _mdfd_getseg's attempts to notice and report an error
 		 * upon access to a missing segment.