[cloud_functions] Correct method channel name on iOS

[izzyaf]

Description

There was a bug fix raised here: #1866. The fix was merged in 0.4.1+8, but then undid by #1707.

This MR is to rename the method channel on iOS to match with the interface.

Related Issues

#1866

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[izzyaf]

@googlebot I signed it!

[googlebot]

CLAs look good, thanks!

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[mark-veenstra]

@collinjackson Can you have a look please? And maybe investigate why it passed the checks again?

[izzyaf]

Superseded by #2051