Update the Golang feature flag example to use Unleash client v3

What does this MR do?

The current example code for Golang pulls an old version of the module which contains an issue that leads to unexpected feature flag values.

Through some troubleshooting in this Slack thread, it was discovered that this has been fixed in version 3 of the library here.

As a result of this change, the example code still works and is now more reliable.

Related issues

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