docs(README.md): improve 'if' clause to detect /pi by knocte · Pull Request #260 · shaftoe/pi-coding-agent-action

Everywhere in this README.md file where the "command" /pi is
explained, the verb "prefix" is used. So then it is making the
user think that the "bot" will only be activated when /pi is
used at the **beginning** of a github comment if one follows the
samples (i.e. in the Quick Start section).

Therefore, it is better to change these samples to use
`startsWith` instead of `contains`. On top of that, to prevent
the potential (although rare) collision with other possible
commands that the user might have set up, or may set up in the
future (e.g. "/pin yadda yadda", or simply "/ping", etc), it's
safer to add a space after "/pi" in the string: "/pi ".