CONTRIBUTING.md: Remove the reference to "Cmd + C"

The "Cmd + C" keyboard shortcut is for copying text,
not quitting programs in the terminal.

The proper way to quit programs in the terminal
under macOS is "Ctrl + C", the same as Linux.

Referring to "Cmd + C" here was based on
a mistaken assumption that "Ctrl" on Windows or Linux
always gets translated to "Cmd" on macOS.

(In fact, some uses of "Ctrl" on Windows/Linux
are preserved as-is on macOS. It's a mixed bag.)

Deleting the reference to "Ctrl + C", to make the guidance clearer.

CONTRIBUTING.md

@@ -84,7 +84,7 @@ In another terminal window, run:
 ```
 docker-compose down
 ```
-_(Shutting down the container in this way is safer than exiting with `Ctrl + C` or `Cmd + C`, and prevents issues with breaking the `db` container.)_
+_(Shutting down the container in this way is safer than exiting with `Ctrl + C`, and prevents issues with breaking the `db` container.)_
 
 ### 8 Optional tasks:
 To clean up encoding problems in the safe2pee data, run (Use `rake db:fix_accents[dry_run]` to preview the changes.):