Add missing link to Ruby guide

Author: felicitymay

docs/codeql/writing-codeql-queries/about-data-flow-analysis.rst

@@ -21,6 +21,7 @@ See the following tutorials for more information about analyzing data flow in sp
 - ":ref:`Analyzing data flow in Java <analyzing-data-flow-in-java>`"
 - ":ref:`Analyzing data flow in JavaScript/TypeScript <analyzing-data-flow-in-javascript-and-typescript>`"
 - ":ref:`Analyzing data flow in Python <analyzing-data-flow-in-python>`"
+- ":ref:`Analyzing data flow in Ruby <analyzing-data-flow-in-ruby>`"
 
 .. pull-quote::