docs: fix unsupported-operation example (reports not-iterable instead)

[Kartikey077]

Summary

Fixes #3796

The unsupported-operation docs example used if "hello" in 1: which
actually reports not-iterable instead of unsupported-operation.

Replaced the example with x = 1 + "oops" which correctly triggers
unsupported-operation.

Test Plan

Verified locally using pyrefly CLI:

Old example:
if "hello" in 1: → reports not-iterable

New example:
x = 1 + "oops" → reports unsupported-operation

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[Kartikey077]

@yangdanny97 could you take a look when you get a chance?
This is a small docs fix for issue #3796. Thank you!

[yangdanny97]

Thank you!