gopls/doc/advanced.md: correct commands for unstable version build
gopls/doc/advanced.md: correct commands for unstable version build

`go get golang.org/x/tools/gopls@master ...` resulted in warning messages like:

go get: installing executables with 'go get' in module mode is deprecated.
        To adjust and download dependencies of the current module, use 'go get -d'.
        To install using requirements of the current module, use 'go install'.
        To install ignoring the current module, use 'go install' with a version,
        like 'go install example.com/cmd@latest'.
        For more information, see https://golang.org/doc/go-get-install-deprecation
        or run 'go help get' or 'go help install'.

Change-Id: I1299983604c27e333f02af875826b5dd4d170068
Reviewed-on: https://go-review.googlesource.com/c/tools/+/388074
Trust: Hyang-Ah Hana Kim <d7e82c3c07f92613bcbf9a1e88f6b96c13caa3b0@gmail.com>
Reviewed-by: Robert Findley <9179eee875d1ee628c37fb47d7e749e855ffd21f@google.com>

golang/tools,golang/tools,golang/tools,golang/tools,golang/tools