Fix a typo in _bt_readpage

I just noticed that e8c334c47a fixes typos in e0b1ee17dc.  I think there
is an omission in _bt_readpage.

--- a/src/backend/access/nbtree/nbtsearch.c
+++ b/src/backend/access/nbtree/nbtsearch.c
@@ -1784,7 +1784,7 @@ _bt_readpage(IndexScanDesc scan, ScanDirection dir, OffsetNumber offnum)
            /*
             * If the result of prechecking required keys was true, then in
             * assert-enabled builds we also recheck that _bt_checkkeys()
-            * result is is the same.
+            * result is the same.
             */
            Assert(!requiredMatchedByPrecheck ||
                   passes_quals == _bt_checkkeys(scan, itup, indnatts, dir,

Thanks
Richard