Uncap 'Publishing Platform'.

Unlike the Performance Platform, which is a known, named thing, the
publishing platform is less formal.

README.md

@@ -1,5 +1,5 @@
 # GOV.UK Content Models
 
-A gem containing the shared models for the GOV.UK Publishing Platform, including [Panopticon](https://github.com/alphagov/panopticon) and [Publisher](https://github.com/alphagov/publisher).
+A gem containing the shared models for the GOV.UK publishing platform, including [Panopticon](https://github.com/alphagov/panopticon) and [Publisher](https://github.com/alphagov/publisher).
 
 This is a continual **work in progress**.