[charmod-norm] fix lingering reference to markup

aphillips has just created a new issue for 
https://github.com/w3c/charmod-norm:

== fix lingering reference to markup ==
>  For example, < or > are part of the markup in an HTML document. 

This should say syntactic content.

See https://github.com/w3c/charmod-norm/issues/22
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