A link in README.md points to a non-existent page

> Please provide us with the following information:
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### This issue is for a: (mark with an `x`)
```
- [ ] bug report -> please search issues before submitting
- [ ] feature request
- [x] documentation issue or request
- [ ] regression (a behavior that used to work and stopped in a new release)
```

### Minimal steps to reproduce
> Follow the link [Microsoft.Identity.Web library](https://github.com/AzureAD/microsoft-identity-web/blob/master/src/Microsoft.Identity.Web/TokenAcquisition.cs#L502) in README.md file

### Any log messages given by the failure
> 404 page not found

### Expected/desired behavior
> Should open a valid page

### OS and Version?
> N/A

### Versions
>

### Mention any other details that might be useful
The link [Microsoft.Identity.Web library](https://github.com/AzureAD/microsoft-identity-web/blob/master/src/Microsoft.Identity.Web/TokenAcquisition.cs#L502) in README.md file is broken and points to a non-existent page.
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