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 <hyangah@gmail.com>
Reviewed-by: Robert Findley <rfindley@google.com>

Author: hyangah

gopls/doc/advanced.md

@@ -16,19 +16,17 @@ Where `vX.Y.Z` is the desired version.
 
 ### Unstable versions
 
-To update `gopls` to the latest **unstable** version, use:
+To update `gopls` to the latest **unstable** version, use the following
+commands.
 
 ```sh
 # Create an empty go.mod file, only for tracking requirements.
 cd $(mktemp -d)
 go mod init gopls-unstable
 
 # Use 'go get' to add requirements and to ensure they work together.
-go get golang.org/x/tools/gopls@master golang.org/x/tools@master
+go get -d golang.org/x/tools/gopls@master golang.org/x/tools@master
 
-# For go1.17 or older, the above `go get` command will build and
-# install `gopls`. For go1.18+ or tip, run the following command to install
-# using selected versions in go.mod.
 go install golang.org/x/tools/gopls
 ```