Checkout update command is spelled two ways: CLI vs the page #1085

On a source checkout, Raven names the update command in two places, and they
disagree:

• raven upgrade prints git pull && ./install.sh on every platform
  (raven/cli/upgrade_commands.py).
• The page's editable refusal from system.upgrade sends git pull then
  ./install.sh, or .\install.ps1 on Windows (raven/rpc/methods/system.py).

On Windows the CLI's spelling is wrong: install.sh stops with "on Windows use
install.ps1". Raised in review of #1003 and left for a follow-up; context in #716.

Expected: one definition in raven/updates/ that both the CLI and the RPC refusal
read, choosing the install script by platform, so the two cannot drift again.