issues: simplify example in ini file

The example setting from the default ini file is:
issue_pat = (?:\s*#)(\d+)
issue_server_link = https://issues.example.com/{repo}/issue/{id}
issue_prefix = #

in which the clause '(?:\s*#)' is overly complex:
1. '\s*' means optional whitespace, which means there can be whitespace or
no whitespace, which means that you could equally write:
(?:#)(\d+)
(If any leading whitespace was eaten by the regexp, 32e1e0745d3c would take
care to restore a part of it.)
2. '(?:xxx)' means a non-capturing set of parentheses. In this case this is
equal to just mentioning 'xxx'. So the simplified pattern becomes:
#(\d+)

If instead of _optional_ whitespace, _mandatory_ whitespace was intended,
then the pattern should be different. But this would also mean that
patterns like PR#123 would not be matched anymore.