docs: fix go install (#376)

#375 (comment)

I found this doesn't work.
To install slsa-verifier v2 by go install, we have to release v2.0.1 or later.

```
go install github.com/slsa-framework/slsa-verifier/v2/cli/slsa-verifier@v2.0.0
go: github.com/slsa-framework/slsa-verifier/v2/cli/slsa-verifier@v2.0.0: github.com/slsa-framework/slsa-verifier@v2.0.0: invalid version: module contains a go.mod file, so module path must match major version ("github.com/slsa-framework/slsa-verifier/v2")
```

Signed-off-by: Shunsuke Suzuki <suzuki.shunsuke.1989@gmail.com>

Signed-off-by: Shunsuke Suzuki <suzuki.shunsuke.1989@gmail.com>

README.md

@@ -120,7 +120,7 @@ You have two options to install the verifier.
 #### Option 1: Install via go
 
 ```
-$ go install github.com/slsa-framework/slsa-verifier/v2/cli/slsa-verifier@v2.0.0
+$ go install github.com/slsa-framework/slsa-verifier/cli/slsa-verifier@v1.3.2
 $ slsa-verifier <options>
 ```