docs(chips): make separatorKeysCodes readonly

[jhuang314]

Make separatorKeysCodes readonly because it isn't changed after component is intialized.

[jelbourn]

Is there a strong reason to do this in the example?

[jhuang314]

In the example, it looks like the key codes are essentially constants, so I think it would make sense if we add the readonly modifier. The motivation for this change proposal came from a teammate who copy pasted the code from the example and interpreted the example code as best practices (i.e. omitting the readonly).

[jelbourn]

LGTM

[ngbot]

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[tinayuangao]

@jhuang314 Could you please rebase? Thanks!

[ngbot]

Hi @jhuang314! This PR has merge conflicts due to recent upstream merges.
Please help to unblock it by resolving these conflicts. Thanks!

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