Fixing broken SLS behaviour

[gregharvey]

The current redmine-3.0 branch requires signout_url and idp_slo_target_url but the callback path doesn't exist, SLS hasn't been implemented on the Redmine side in this branch. Looks like it was added for 4.x. Consequently, logout cannot succeed but you also cannot make Redmine use local logout (even though the code is actually there to do this).

So this PR does three things:

1. Stops signout_url and idp_slo_target_url being required for validation (SAML does not require them)
2. Fixes a little typo in the sign-out method which causes an error
3. Updates the sample documentation according to what I've discovered the settings should be

Hope this helps! :-)