[doc] minor adjust create-mime.conf.pl regex match (#2942)
[doc] minor adjust create-mime.conf.pl regex match (#2942)

'-' in char class should be at beginning or end, or escaped with \-
so that it does not signify a char range in the char class

(In the prior regex, the misuse permitted matching the comma char,
which while mostly harmless, was not what was intended)

x-ref:
  "incorrect behaviour of create-mime.assign.pl"
  https://redmine.lighttpd.net/issues/2942

lighttpd/lighttpd1.4,gstrauss/lighttpd1.4,lighttpd/lighttpd1.4,lighttpd/lighttpd1.4,lighttpd/lighttpd1.4,gstrauss/lighttpd1.4,gstrauss/lighttpd1.4,gstrauss/lighttpd1.4,gstrauss/lighttpd1.4,lighttpd/lighttpd1.4,lighttpd/lighttpd1.4,gstrauss/lighttpd1.4