Fix grammar in package-summary.mdx (#2308)

* Fix grammar in package-summary.mdx

Fixed double `which` and converted `customise` to `customize` to be consistent with this PR: #1870

* fix: another customize fix

Author: flugg

docs/package-summary.mdx

@@ -17,11 +17,11 @@ Depends on @emotion/react
 ## @emotion/css
 
 Contains a framework agnostic version of Emotion. Has different functionality and APIs from @emotion/react and doesn’t work the same way under the hood.
-Depends on @emotion/cache which can be customised by creating a custom instance at @emotion/css/create-instance
+Depends on @emotion/cache which can be customized by creating a custom instance at @emotion/css/create-instance
 
 ## @emotion/cache
 
-A cache used by both of the main Emotion APIs. Can be used to provide a custom cache in @emotion/react with its CacheProvider. To customise the equivalent options for @emotion/css, @emotion/css/create-instance can create another instance of @emotion/css and it accepts
+A cache used by both of the main Emotion APIs. Can be used to provide a custom cache in @emotion/react with its CacheProvider. To customize the equivalent options for @emotion/css, @emotion/css/create-instance can create another instance of @emotion/css and it accepts
 Depended upon by @emotion/react and @emotion/css
 
 ## @emotion/babel-plugin