globbing: fix step parsing for character globbing ranges

The glob_range function used wrong offset (3 instead of 4) for parsing
integer step inside character range specification, which led to 'bad
range' error when using character ranges with explicitly specified step
(such as '[a-z:2]')

src/tool_urlglob.c

@@ -196,7 +196,7 @@ static CURLcode glob_range(URLGlob *glob, char **patternp,
       char *endp;
       unsigned long lstep;
       errno = 0;
-      lstep = strtoul(&pattern[3], &endp, 10);
+      lstep = strtoul(&pattern[4], &endp, 10);
       if(errno || (*endp != ']'))
         step = -1;
       else {