[func.wrap.func.targ] Use 'class' not 'typename'

The rest of the library always uses 'class' in template parameter lists,
so change the only occurrence of 'typename' to be consistent.
Also fix tcode{} markup to include entire expression.
Merge pull request #44 from jwakely/func.wrap.func.targ

[func.wrap.func.targ] Use 'class' not 'typename' in template parameter lists. Thanks to Jonathan Wakely!