Fix Azure AD B2C markdown link (dotnet#18232)

aspnetcore/includes/IdentityServer4.md

@@ -1,7 +1,7 @@
 ASP.NET Core Identity adds user interface (UI) login functionality to ASP.NET Core web apps. To secure web APIs and SPAs, use one of the following:
 
 * [Azure Active Directory](/azure/api-management/api-management-howto-protect-backend-with-aad)
-* [Azure Active Directory B2C](/azure/active-directory-b2c/active-directory-b2c-custom-rest-api-netfw) (Azure AD B2C)]
+* [Azure Active Directory B2C](/azure/active-directory-b2c/active-directory-b2c-custom-rest-api-netfw) (Azure AD B2C)
 * [IdentityServer4](https://identityserver.io)
 
 IdentityServer4 is an OpenID Connect and OAuth 2.0 framework for ASP.NET Core 3.0. IdentityServer4 enables the following security features:
@@ -11,4 +11,4 @@ IdentityServer4 is an OpenID Connect and OAuth 2.0 framework for ASP.NET Core 3.
 * Access control for APIs
 * Federation Gateway
 
-For more information, see [Welcome to IdentityServer4](http://docs.identityserver.io/en/latest/index.html).
+For more information, see [Welcome to IdentityServer4](http://docs.identityserver.io/en/latest/index.html).