doc: add while loops to sample file

doc/sample/Expressions.im

@@ -48,3 +48,9 @@ Q = match yz in
 	3 => age = age / 365,
 	_ => age = 0
 end.
+
+while x > 2 do
+	cout put:"message"
+end
+
+cout put:"message" while x > 2.