update rainbow.txt

modules/readmes/rainbow.txt

@@ -1,9 +1,11 @@
 in the css
 
-@import url("https://styles.alv.cx/modules/rainbow.css");
+	@import url("https://styles.alv.cx/modules/rainbow.css");
+
+variables:
+
+	--rainbow-duration: 8s;
 
 and add the class `rainbow` to any element you want.
 
 it works by changing the `color` property of the element.
-
-the duration is determined by the variable `--rainbow-duration`