escape curly braces for parsing @custom attribute

[skurzinz]

This avoids the following saxon error and a few more:

Error evaluating (fn:tokenize(...)) in xsl:for-each-group/@select on line 217 column 118 of page2tei-0.xsl: FORX0002: Syntax error at char 0 in regular expression: Unescaped right curly brace

[bertsky]

I can confirm it is necessary to escape the right curly brace in the second argument to the tokenize function with SaxonHE10-6J. (Not sure about the other escapes here, though.)