Amplify add examples for SAML provider

**Describe the content issue**:

Hello Team,

In the following documentation, the examples are provided for social providers, but there is no mention that the same syntax would work for SAML provider as well. This confuses customers sometimes to assume that there is a different syntax for SAML.

[+] https://docs.amplify.aws/lib/auth/social/q/platform/js/#setup-frontend

For SAML we can add the following example. I am able to test and confirm that both the below works for using SAML provider
Auth.federatedSignIn({customProvider: 'SAMLProviderName' })
Auth.federatedSignIn({provider: 'SAMLProviderName' })