Rules shall enforce things

[xbonnetain]

trafficstars

Rule 5 is currently written as a suggestion:

The tool rustfmt can be used to ensure...

This is the only rule written this way, and it defeats its purpose.

This PR changes "can" to "must".

Other approaches

There are other possibilities, for example:

• Removing this rule
• Adding a "Suggestion/Best practice" block for non-mandatory things

[danielhenrymantilla]

Forcing the usage of a specific formatter feels contrived, though. Maybe "an autoformatting tool (e.g. rustfmt) must be used". This way the phrasing remains open to alternative formatters that may arise.