attempt to fix xslt error

```
2014-02-07 15:31:01,435 [ERROR] com.xmlcalabash - bundle://149.1:1/xml/xslt/deep-level-grouping.xsl:234:err:XTSE0090:Attribute @required is not allowed on element <xsl:param>
2014-02-07 15:31:01,444 [ERROR] com.xmlcalabash - Failed to compile stylesheet. 1 error detected.
```