MSVC: fix compiler error in GenericDocument

The `StringRefType` assignment operator overload
leads to a compiler error on MSVC 2005 and later:

..\..\include\rapidjson/document.h(504) : error C2951: template declarations are only permitted at global, namespace, or class scope

Drop the unneeded 'template' keyword here.

include/rapidjson/document.h

@@ -501,7 +501,7 @@ public:
 		\see GenericStringRef, operator=(T)
 	*/
 	GenericValue& operator=(StringRefType str) {
-		return (*this).template operator=<StringRefType>(str);
+		return (*this).operator=<StringRefType>(str);
 	}
 
 	//! Assignment with primitive types.