Add "String Formatting" tutorial link to the 'String' doc

(cherry picked from commit 3197898c88)

doc/classes/String.xml

@@ -7,6 +7,7 @@
 		This is the built-in string class (and the one used by GDScript). It supports Unicode and provides all necessary means for string handling. Strings are reference counted and use a copy-on-write approach, so passing them around is cheap in resources.
 	</description>
 	<tutorials>
+		<link>https://docs.godotengine.org/en/3.1/getting_started/scripting/gdscript/gdscript_format_string.html</link>
 	</tutorials>
 	<methods>
 		<method name="String">