Clarify 2FA QR scanning instructions

What does this MR do?

When I follow the instructions to set up 2FA, I get the following instructions:

Install a soft token authenticator like FreeOTP or Google Authenticator from your application repository and scan this QR code. More information is available in the documentation.

I immediately think "Find my QR code reader" -- when what I actually need to do is use the QR reader from the selected authenticator.

So I propose adding "use that app" to the instructions.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team