Change to GB spelling and disable checking of internal refs

The spellchecker will catch issues in the reference html output.
Internal references found in other pages do not need to be checked also.
This raises unecessary spellcheck errors otherwise.

docs/.sphinx/spellingcheck.yaml

@@ -2,7 +2,7 @@ matrix:
 - name: rST files
   aspell:
     lang: en
-    d: en_US
+    d: en_GB
   dictionary:
     wordlists:
     - .wordlist.txt
@@ -28,3 +28,4 @@ matrix:
       - img
       - a.p-navigation__link
       - a.contributor
+      - a.reference.internal