message: gzip filter: fix docs wrt runtime feature flag
  Althought this filter is deprecated in favor of the compressor
  filter, let's go ahead and fix the incorrect reference to the
  now optional runtime feature flag.
diff: @@ -29,11 +29,9 @@ Configuration
  Runtime
  -------
  -The Gzip filter supports the following runtime settings:
  -
  -gzip.filter_enabled
  - The % of requests for which the filter is enabled. Default is 100.
  -
  +The Gzip filter can be runtime feature flagged via the :ref:`runtime_enabled
  +<envoy_v3_api_field_extensions.filters.http.gzip.v3.Gzip.compressor>`
  +configuration field within the compressor field.
  How it works
  ------------
  