docs: Fixed wrong function URL in README

[crmne]

A simple pull request that fixes the problem I had while trying cloud functions locally that connect to the pubsub emulator.

Before, it used to point the subscription to the pubsub emulator itself, instead of the cloud function endpoint.

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[crmne]

@googlebot I signed it!

[di]

@anguillanneuf Can you review?

@crmne You'll need to add docs: to your commit message to get the "Conventional Commits" check to pass.