xml.aug: Allow nmotoken rather than plain word after !DOCTYPE

Currently, the lens only accepts a word after the `!DOCTYPE` keyword.

A - valid - doctype can have the following form :

```xml
<!DOCTYPE log4j:configuration SYSTEM "log4j.dtd">
```
making the xml file invalid with the current lens. This commits aims for the lens
to accept nmtoken rather than strict word after the `!DOCTYPE` keyword.