Update repl README's standalone build commands

[davidjb]

By default, go build . was outputting ./main as a binary, meaning the latter mv command doesn't work. This sets an output flag from go build such that repl gets built. Conversely, the mv line could be updated (and would be consistent with the main.go filename), but an executable called main seems too generic.

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[TristonianJones]

Thanks for fixing this!