Link to the Random number generation tutorial in RandomNumberGenerator

This was done in `master` already, but not in `3.x`.

(cherry picked from commit 93f4f9e076)

doc/classes/RandomNumberGenerator.xml

@@ -16,6 +16,7 @@
 		[b]Note:[/b] The default values of [member seed] and [member state] properties are pseudo-random, and changes when calling [method randomize]. The [code]0[/code] value documented here is a placeholder, and not the actual default seed.
 	</description>
 	<tutorials>
+		<link title="Random number generation">https://docs.godotengine.org/en/3.3/tutorials/math/random_number_generation.html</link>
 	</tutorials>
 	<methods>
 		<method name="randf">