doc: Fix use of SAML2_Const in documentation.

Since we have updated the version of the SAML2 library in use, we should use SAML2\Constants now.

modules/saml/docs/sp.md

@@ -55,7 +55,7 @@ All these parameters override the equivalent option from the configuration.
 `saml:NameID`
 :   Add a Subject element with a NameID to the SAML AuthnRequest for the IdP.
     This is an associative array with the fields for the NameID.
-    Example: `array('Value' => 'user@example.org', 'Format' => SAML2_Const::NAMEID_UNSPECIFIED)`
+    Example: `array('Value' => 'user@example.org', 'Format' => SAML2\Constants::NAMEID_UNSPECIFIED)`
 
 :   *Note*: SAML 2 specific.