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.

development.ini

@@ -175,7 +175,7 b' default_encoding = utf8'
 ## default one used here is #<numbers> with a regex passive group for `#`
 ## {id} will be all groups matched from this pattern
 
-issue_pat = (?:\s*#)(\d+)
+issue_pat = #(\d+)
 
 ## server url to the issue, each {id} will be replaced with match
 ## fetched from the regex and {repo} is replaced with full repository name

kallithea/lib/paster_commands/template.ini.mako

@@ -268,7 +268,7 b' default_encoding = utf8'
 <%text>## default one used here is #<numbers> with a regex passive group for `#`</%text>
 <%text>## {id} will be all groups matched from this pattern</%text>
 
-issue_pat = (?:\s*#)(\d+)
+issue_pat = #(\d+)
 
 <%text>## server url to the issue, each {id} will be replaced with match</%text>
 <%text>## fetched from the regex and {repo} is replaced with full repository name</%text>