Fix reference to attribute, _key-name (#83)

Repairing apparent error in converter-generator XSLT otherwise undetected

src/converter-gen/xml-to-json/produce-xml-converter.xsl

@@ -81,7 +81,7 @@
             </xsl:for-each-group>
 
             <!-- again, this time for (all) flags, grouping by key name when available -->
-            <xsl:for-each-group select=".//flag" group-by="(@key-name,@_metaschema-xml-id)[1]">
+            <xsl:for-each-group select=".//flag" group-by="(@_key-name,@_metaschema-xml-id)[1]">
                 <xsl:apply-templates select="current-group()[1]" mode="make-template">
                     <xsl:with-param name="team" tunnel="true" select="current-group()"/>
                 </xsl:apply-templates>