Fix build of pg_verify_checksum docs

They were accidentally excluded when reverting the backend online
checksum functionality, and since they weren't built the incorrect
reference to a removed section also did not trigger a problem.

Author: Christoph Berg

doc/src/sgml/reference.sgml

@@ -255,6 +255,7 @@
    &pgReceivewal;
    &pgRecvlogical;
    &pgRestore;
+   &pgVerifyChecksums;
    &psqlRef;
    &reindexdb;
    &vacuumdb;