Re-fix broken definition for function name in pgbench's exprscan.l.

Wups, my first try wasn't quite right either.  Too focused on fixing
the existing bug, not enough on not introducing new ones.

src/bin/pgbench/exprscan.l

@@ -61,7 +61,7 @@ space			[ \t\r\f]
 					yylval.ival = strtoint64(yytext);
 					return INTEGER;
 				}
-{alpha}{alnum}+	{
+{alpha}{alnum}*	{
 					yycol += yyleng;
 					yylval.str = pg_strdup(yytext);
 					return FUNCTION;