Fixed documentation for gopls installation

[dcormier]

Fixes this problem when trying to use the documented installation command:

$ GO111MODULE=on go get golang.org/x/tools gopls@latest
go: finding golang.org/x/tools latest
go: downloading golang.org/x/tools v0.0.0-20190911230505-6bfd74cf029c
go: extracting golang.org/x/tools v0.0.0-20190911230505-6bfd74cf029c
go get gopls@latest: malformed module path "gopls": missing dot in first path element

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Message from Ian Cottrell:

Patch Set 1: Code-Review+2

Thanks!

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This PR is being closed because golang.org/cl/195037 has been merged.