Need to update module path in go.mod to v2

While there is a tag and release for v2.0.0, this is not reflected in go.mod.

This means that only v1 can be imported, as golang requires modules to reflect major versions in go.mod.

Open go.mod and compare its module path with the existing v2.0.0 tag and release. Update the module path to use the required v2 major-version suffix, then verify that the v2 module can be imported as described.

索引モデルが issue の本文から書いたものです。