fix indentation of closing xml tag

There's a regression in the xml mode that prevents the correct indentation of closing tags. This only happens if your tag contains characters that are not word characters, e.g. "-", or ":". Those are supported in XML 1.0 (and 1.1)

The existing regexp in mode/xml/xml.js line 337 would fail to correctly detect the closing tag &lt;/hello-world&gt;  so tagAfter[2] gets the value "hello" instead of "hello-world"