typo fix in pgstat.c: "exits should be exists"

Hi,

While reviewing replication statistics I found a small typo. Attached
patch for a typo in:
src/backend/postmaster/pgstat.c
................
                /*
-                * Check if the slot exits with the given name. It is
possible that by
+                * Check if the slot exists with the given name. It is
possible that by
                 * the time this message is executed the slot is
dropped but at least
                 * this check will ensure that the given name is for a
valid slot.
                 */
................

Regards,
Vignesh