CONTRIBUTING.md

When using
dns_upstream_forward_extra_opts:
  prefer_udp: "" # the option as no value so use empty string to just
                 # put the key

This is rendered in the dns configmap as ($ for end-of-line)

...
  prefer_udp $
...

Note the trailing space.
This triggers https://github.com/kubernetes/kubernetes/issues/36222,
which makes the configmap hardly readable when editing them manually or
simply putting them in a yaml file for inspection.

Trim the concatenation of option + value to get rid of any trailing
space.