fix sign-in sign-up on B2C project by adding appsetting value

[chuck-waters]

Fixes issue #669 for the B2C project example

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Summary of the changes (Less than 80 chars)

Description

Issue with the B2C example with the sign-in sign-up policy. Adding a key called "Tenant" with the same value as the domain seems to fix the issue.

Fixes #669 - missing appsettings value

[chuck-waters]

@microsoft-github-policy-service agree

[chuck-waters]

I can't seem to add or request a review. Something you can help with @jmprieur ?