Update "How Should I reference the name ATT&CK" in FAQ

[isaisabel]

As a user of the ATT&CK website, I want to see accurate legal language reflecting the status of the ATT&CK trademark.

The FAQ section, "How Should I reference the name ATT&CK", needs to be updated to reflect that ATT&CK is now a registered trademark.

[isaisabel]

@IanDavila, the following are the relevant changes to that FAQ section:

• changing the ™ to ® wherever it occurs
• Both MITRE ATT&CK™ and ATT&CK™ are trademarks of The MITRE Corporation. becomes Both MITRE ATT&CK® and ATT&CK® are registered trademarks of The MITRE Corporation.

Everything else can remain the same.