docs: update import statement for app_role_assignment

[tberreis]

This small PR updates the import statement for the azuread_app_role_assignment.
I noted a change when recently using it.

Error: parsing "d00*****-****-****-****-*****39b503c/appRoleAssignment/Jno_******************************NWf5jBrA4": parsing the ServicePrincipalIdAppRoleAssignedTo ID: the number of segments didn't match
│ 
│ Expected a ServicePrincipalIdAppRoleAssignedTo ID that matched (containing 4 segments):
│ 
│ > /servicePrincipals/servicePrincipalId/appRoleAssignedTo/appRoleAssignmentId
│ 
│ However this value was provided (which was parsed into 0 segments):
│ 
│ > d00*****-****-****-****-*****39b503c/appRoleAssignment/Jno_******************************NWf5jBrA4
│ 
│ The following Segments are expected:
│ 
│ * Segment 0 - this should be the literal value "servicePrincipals"
│ * Segment 1 - this should be the user specified value for this servicePrincipalId [for example "servicePrincipalId"]
│ * Segment 2 - this should be the literal value "appRoleAssignedTo"
│ * Segment 3 - this should be the user specified value for this appRoleAssignmentId [for example "appRoleAssignmentId"]
│ 
│ The following Segments were parsed:
│ 
│ * Segment 0 - not found
│ * Segment 1 - not found
│ * Segment 2 - not found
│ * Segment 3 - not found