Fix syntax error in README code sample

[KevinTCoughlin]

Summary

There's a syntax error in the MSAL configuration code sample that prevents folks from running the code when copy & pasted into their solution.

I ran into it while onboarding to the library, specifically when trying to use ImplicitMSALAuthenticationProvider.

cc: @natalieethell

Motivation

This is a minor documentation change.

Test plan

N/A

Closing issues

N/A

Types of changes

• Documentation
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• I have read the CONTRIBUTING document.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.