Doc: fix typos.

"PGcon" should be "PGconn".  Noted by D. Frey.

Discussion: https://postgr.es/m/163191739352.4680.16994248583642672629@wrigleys.postgresql.org

doc/src/sgml/lobj.sgml

@@ -453,7 +453,7 @@ pg_int64 lo_tell64(PGconn *conn, int fd);
      <indexterm><primary>lo_truncate</primary></indexterm>
      To truncate a large object to a given length, call
 <synopsis>
-int lo_truncate(PGcon *conn, int fd, size_t len);
+int lo_truncate(PGconn *conn, int fd, size_t len);
 </synopsis>
      This function truncates the large object
      descriptor <parameter>fd</parameter> to length <parameter>len</parameter>.  The
@@ -481,7 +481,7 @@ int lo_truncate(PGcon *conn, int fd, size_t len);
      When dealing with large objects that might exceed 2GB in size,
      instead use
 <synopsis>
-int lo_truncate64(PGcon *conn, int fd, pg_int64 len);
+int lo_truncate64(PGconn *conn, int fd, pg_int64 len);
 </synopsis>
      This function has the same
      behavior as <function>lo_truncate</function>, but it can accept a