Update installer README, fix deprecated snippet

[matt-savvy]

Description

The installer README uses a deprecated mix do feature in the "build and install locally" instructions.

The existing snippet gives this warning

warning: using commas as separators in "mix do" is deprecated, use + between commands instead
  (mix 1.19.3) lib/mix/tasks/do.ex:129: Mix.Tasks.Do.gather_commands/3
  (mix 1.19.3) lib/mix/tasks/do.ex:75: Mix.Tasks.Do.run/1
  (mix 1.19.3) lib/mix/task.ex:499: anonymous fn/3 in Mix.Task.run_task/5
  (mix 1.19.3) lib/mix/cli.ex:129: Mix.CLI.run_task/2
  /usr/local/bin/mix:7: (file)
  (elixir 1.19.3) src/elixir_compiler.erl:81: :elixir_compiler.dispatch/4

Changes

Update installer README, fix deprecated snippet.

[SteffenDE]

Thank you! 🙌🏻