Fix grammar in package-summary.mdx

[flugg]

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

Why:
Makes it easier to read.

How:
By using the keyboard.

Checklist:

• Documentation
• Tests
• Code complete
• Changeset

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Latest commit: 9e96d42

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Emotion	Configuration

[flugg]

There also seems to be an incomplete sentence in the @emotion/cache paragraph:

...can create another instance of @emotion/css and it accepts
Depended upon by @emotion/react and @emotion/css

Not sure what it accepts. If it's wanted, feel free to throw me a full sentence and I can add it to this PR.

[Andarist]

both "customized" and "customised" are correct - the former is used by American English and the latter is British.

@mitchellhamilton do you have any preference over this? to which flavor Australian is closer?

[emmatown]

I don't really have a strong preference, American is probably the better one to use to be consistent with what APIs generally use (e.g. color in CSS).