[macros] Fix duplicate vertical lines visible in tables in [optional.assign]

Fixes ISO/CS comment (C++23 proof)

Author: jensmaurer

source/tables.tex

@@ -492,7 +492,7 @@
  \newcommand{\bottomline}{\rowsep}
  \newcommand{\hdstyle}[1]{\textbf{##1}}
  \newcommand{\rowhdr}[1]{\hdstyle{##1}&}
- \newcommand{\colhdr}[1]{\multicolumn{1}{|>{\centering}m{#6}|}{\hdstyle{##1}}}
+ \newcommand{\colhdr}[1]{\multicolumn{1}{>{\centering}m{#6}|}{\hdstyle{##1}}}
  \begin{floattablebasex}
  {#1}{#2}
  {>{\centering}m{#5}|@{}p{0.2\normalbaselineskip}@{}|m{#6}|m{#7} }