Update the docs site to reflect updated README

[finnlewis]

• https://github.com/localgovdrupal/localgov/commit/800ca3c82b0ac8d9d6f6ce065afc2a8410ba8769

• https://docs.localgovdrupal.org/devs/getting-started/

the docs site instructions (composer command) is now out of date.

[finnlewis]

I think you've already done this @msayoung !