fix regex alternation

[geseq]

fixes #459

[geseq]

@d5 can you review please?

[lxzyork]

@geseq Hi geseq, I'm having trouble importing the tengo package v3 in go.mod, getting the following error.

$ go get github.com/d5/tengo@3.0.0
go: errors parsing go.mod:
go.mod:10:2: require github.com/d5/tengo/v3: version "v3.0.0" invalid: go.mod has non-.../v3 module path "github.com/d5/tengo/v2" (and .../v3/go.mod does not exist) at revision v3.0.0

What steps should I take? Thank you for your guidance.