Update translation textdomain per WP standards

• Fixes #18 (closed)
• Adds Polish translation care of [Maciej Gryniuk], submitted in https://github.com/ethitter/WP-Revisions-Control/pull/5

phpcs.xml

@@ -38,7 +38,7 @@
 	<rule ref="WordPress.WP.I18n">
 		<properties>
 			<!-- Value: replace the text domain used. -->
-			<property name="text_domain" type="array" value="wp_revisions_control"/>
+			<property name="text_domain" type="array" value="wp-revisions-control"/>
 		</properties>
 	</rule>
 	<rule ref="WordPress.WhiteSpace.ControlStructureSpacing">