Update concrete-immutable-collection-classes.md

[AbnerZheng]

常熟->常数

[heathermiller]

Hi @AbnerZheng thanks for your contribution! In theory this LGTM, I admit my Chinese is a bit rusty :P I don't know of any Chinese-speakers off-hand that I could ask to review this.

Is there someone you could suggest that could have a look?

[SethTisue]

@apapi, interested in reviewing?

[som-snytt]

It's a clever pun on "constant time" and "the time is ripe". Unfortunately, my spouse, who hates all my puns, is visiting in China this week. But google says, 对我来说很好。

[SethTisue]

I asked for a reviewer on scala/contributors. If nobody turns up I guess we should just merge

[SethTisue]

@marky0720 interested in reviewing this?

[som-snytt]

LGTM

OK, my wife is back from China yesterday. She confirms the fix. She also got my pun; or possibly is just being nice about it.

[floating-cat]

I am a Chinese. This PR is just a typo fix. You can merge this commit without any worries.

[heathermiller]

Thanks all!