Add-DbaAgDatabase: Fix spelling - use JoinAvailablityGroup with the typo as this is what the SMO uses

[andreasjordan]

Type of Change

• Bug fix (non-breaking change, fixes Add-DbaAgDatabase - Method invocation failed because [Microsoft.SqlServer.Management.Smo.AvailabilityDatabase] does not contain a method named 'JoinAvailabilityGroup'. #9791, reverts Availability is misspelled #9757 )
• New feature (non-breaking change, adds functionality, fixes # )
• Breaking change (affects multiple commands or functionality, fixes # )
• Ran manual Pester test and has passed (Invoke-ManualPester)
• Adding code coverage to existing functionality
• Pester test is included
• If new file reference added for test, has is been added to github.com/dataplat/appveyor-lab ?
• Unit test is included
• Documentation
• Build system

[potatoqualitee]

Whoa! Thank you. I would not have assumed this. Also wondering why our tests pas.