Update Register-SPMTMigration.md

[JoanneHendrickson]

Adding note to the usage of the -SPOCredential parameter when using multifactor authentication.

[opbld33]

Docs Build status updates of commit 43c6d6e:

🕙 Full build: the average full build time is 11 min(s) 47 sec(s), this is based on the last 1000 build(s) for this repository.

[opbld30]

Docs Build status updates of commit 43c6d6e:

✅ Validation status: passed

File	Status	Preview URL	Details
spmt/spmt-ps/spmt/Register-SPMTMigration.md	✅Succeeded	View (spmt-ps)

For more details, please refer to the build report.

Note: If you changed an existing file name or deleted a file, broken links in other files to the deleted or renamed file are listed only in the full build report.

[opbld34]

Docs Build status updates of commit 7af0785:

🕙 Full build: the average full build time is 11 min(s) 47 sec(s), this is based on the last 1000 build(s) for this repository.

[opbld30]

Docs Build status updates of commit 7af0785:

✅ Validation status: passed

File	Status	Preview URL	Details
spmt/spmt-ps/spmt/Register-SPMTMigration.md	✅Succeeded	View (spmt-ps)

For more details, please refer to the build report.

Note: If you changed an existing file name or deleted a file, broken links in other files to the deleted or renamed file are listed only in the full build report.