override namespace Observable #trivial

[trmquang93]

this resolve error on Xcode 15, that can not build project

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[JoeMatt]

I haven't seen an error myself.

Could this cause issues with older XCode. I don't know what the RxSwift community guidelines are on something like that, if there are any.

This change looks sound and simple to me but I want to double check it won't give another project problems.