Add Ruby snippet for calculating factorial

Author: ACR1209

snippets/ruby/math-and-numbers/calculate-factorial.md

@@ -0,0 +1,16 @@
+---
+title: Calculate Factorial
+description: Computes the factorial of a given integer.
+author: ACR1209
+tags: ruby,math,factorial
+---
+
+```rb
+def factorial(n)
+    return 1 if n <= 1
+    (2..n).reduce(1, :*)
+end
+
+# Usage:
+puts factorial(5) # Output: 120
+```