Two advantages:
- higher relevance of the extremely common word "heredocs" which may
help people find this page when searching for "ruby heredocs"
- the anchor link becomes `#label-Here+Documents+-28heredocs-29`, which is
ugly due to the parentheses but includes the word "heredocs" in the URL to
this section

If anyone knows a way to prevent RDoc from turning invalid characters into
ugly and meaningless ASCII codes, I'm listening. I don't want to break existing
anchor links but RDoc should really ignore these characters or turn them into
dashes.

Closes: https://github.com/ruby/ruby/pull/2103