psql: print longtable as a possible \pset option

For some reason this message was not updated when the longtable option
was added.

Backpatch through 9.3

src/bin/psql/command.c

@@ -2486,7 +2486,7 @@ do_pset(const char *param, const char *value, printQueryOpt *popt, bool quiet)
 			popt->topt.format = PRINT_TROFF_MS;
 		else
 		{
-			psql_error("\\pset: allowed formats are unaligned, aligned, wrapped, html, asciidoc, latex, troff-ms\n");
+			psql_error("\\pset: allowed formats are unaligned, aligned, wrapped, html, asciidoc, latex, latex-longtable, troff-ms\n");
 			return false;
 		}