docs: use British English throughout (#3336)

fluxer_docs/src/content/docs/admin-api/instance.mdx

@@ -188,7 +188,7 @@ How often a client polls [Get experiment assignments](/http-api/experiments/#get
 
 Every field is present on read. An absent document or missing field uses the defaults above.
 
-Both fields are served to every account, whether or not any experiment targets that account, and neither one is versioned by `config_version`. A client that has never received a response from the experiments route uses built-in defaults of 300 seconds and 15 percent, which equal the defaults above. A client that cannot read the route never receives either field.
+Both fields are served to every account, whether or not any experiment targets that account, and neither one is versioned by `config_version`. A client that has never received a response from the experiments route uses built-in defaults of 300 seconds and 15 per cent, which equal the defaults above. A client that cannot read the route never receives either field.
 
 ## Registration configuration object