README: Update the Patch Headers section (jsc#PED-5021)
    
    * Fix typos, slightly improve some wording and avoid writing additional
      details in parentheses.
    * Remove ":" from the names of patch tags which appear in regular
      sentences. The suffix is somewhat redundant and made README
      inconsistent with doc/README.PATCH-POLICY.SUSE in this regard.
    * Provide an updated example for the patch header format. The new
      example is shorter and shows current typically-used references.