Improve messages for SelectorFormat

I was getting the message that my selector 'should be written must match
regex ...'; so I looked into the file and tidied up those strings, in
order to output consistently grammatical messages.

Change-Id: I862ad36ce2365ac3492252b229c4014505d983eb
Reviewed-on: http://gerrit.causes.com/45265
Tested-by: jenkins <jenkins@brigade.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>
Update changelog with grammar fix

Change-Id: I1d7e7d9dd1e5c63198fd1d7f476c7bd5eefb96c3
Reviewed-on: http://gerrit.causes.com/45266
Tested-by: jenkins <jenkins@brigade.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>