[range.concat.iterator] Add missing \tcode for difference_type (#6943)

Author: hewillk

source/ranges.tex

@@ -9314,7 +9314,7 @@
 \range{$i_\tcode{y}$ + 1}{$i_\tcode{x}$}
 if there is any, and
 \tcode{0} otherwise,
-of type difference_type,
+of type \tcode{difference_type},
 equivalent to:
 \begin{codeblock}
 return @$d_\tcode{y}$@ + @$s$@ + @$d_\tcode{x}$@;