System information:

• system-type: gnu/linux
• locale-coding-system: utf-8-unix
• emacs-version: GNU Emacs 26.1 (build 1, x86_64-Ubuntu-linux-gnu, X toolkit, Xaw scroll bars)
  of 2019-03-12
• markdown-toc version: 0.1.2

heading contains underscore _:

# under_socre

github generate link #under_score
but markdown-toc generate - [under_score][#undersocre].

Because _ is matched [[:punct:]] so replace into "" in markdown-toc--to-link.