Fix build instructions for Go 1.18+ (google#412)

Reference: https://golang.org/doc/go-get-install-deprecation

README.md

@@ -53,7 +53,7 @@ Build the code:
 ```shell
 # check out the code and dependencies,
 # and install interpreter in $GOPATH/bin
-$ go get -u go.starlark.net/cmd/starlark
+$ go install go.starlark.net/cmd/starlark@latest
 ```
 
 Run the interpreter: