DocLinkChecker: using _ in headers is a valid character in link but not in checker

When you have a header with an underscore and reference the header, the link is not recognized. Example:

# This is a header with something_else in it

The [link](#this-is-a-header-with-something_else-in-it) throws an error.

DocLinkChecker expects #this-is-a-header-with-something-else-in-it, which is wrong.