Fix invalid regex in scripts/python/md-split.py

[jwakely]

The [ character doesn't need to be escaped.

The ] character needs to be escaped as \\] or as \] in a raw string.

This fixes warnings seen in the CI checks:

##################### Tabs check ##################
/home/runner/work/CppCoreGuidelines/CppCoreGuidelines/scripts/./python/md-split.py:184: SyntaxWarning: invalid escape sequence '\['
  sline = re.sub('[()\[\]#*]', ' ', line)