Fix installation instruction for the latest release

[alexandear]

This PR fixes installation instruction for the latest release in README.

Without @latest you get:

❯ go install github.com/bombsimon/wsl/v4/cmd/wsl

go: 'go install' requires a version when current directory is not in a module
        Try 'go install github.com/bombsimon/wsl/v4/cmd/wsl@latest' to install the latest version

[coveralls]

coverage: 93.637%. remained the same
when pulling 1f4dd73 on alexandear:fix-install-readme
into 04c20ac on bombsimon:master.

[alexandear]

The fix for CI in the #151