Update signin_external_providers.rst (#4767)
Update signin_external_providers.rst (#4767)

SignInAsync extension methods seems to have changed, the one in the docs here doesn't work. Changed line 131 to new method.
IdentityServer/IdentityServer4,MienDev/IdentityServer4,MienDev/IdentityServer4,IdentityServer/IdentityServer4,MienDev/IdentityServer4,IdentityServer/IdentityServer4,IdentityServer/IdentityServer4,MienDev/IdentityServer4