Fix typo: require_executable -> required_executable

[jfly]

It looks like this incorrect field name has existed since the inception
of the Stylelint plugin in
22229c9.

[sds]

Thanks!

[ChrisBr]

@sds this triggers to update the signature. Maybe this should be part of the CHANGELOG because that's something people should know about, no?

[sds]

Hey @ChrisBr, thanks for catching. This was an oversight.

Updated change log in 86ef690.

[ChrisBr]

Thanks @sds 👍