Fix go get

[wlkh]

Thanks for the module, it looks great!

Currently go get fails due to the path in the go.mod file:

go: downloading github.com/exyzzy/hershey v0.0.0-20210918043116-4417c90c656f
go: github.com/exyzzy/hershey@v0.0.0-20210918043116-4417c90c656f: parsing go.mod:
	module declares its path as: bitbucket.org/exyzzy/hershey
	        but was required as: github.com/exyzzy/hershey

This patch should fix that issue.