Add leisure=park to POIs layer

Given the other leisure=* tags in the POI layer omitting
leisure=park appears to be an oversight.

Author: pnorman

shortbread-website/content/schema/1.1.md

@@ -751,6 +751,7 @@ The following key-value combinations are included in this layer:
 - {{< tag leisure dog_park >}}
 - {{< tag leisure golf_course >}}
 - {{< tag leisure ice_rink >}}
+- {{< tag leisure park >}}
 - {{< tag leisure pitch >}}
 - {{< tag leisure playground >}}
 - {{< tag leisure sports_centre >}}