Do not use "rule of thumb"

This is extremely minor, but "the rule of thumb" meant that a man could beat his wife and children with a stick roughly the diameter of his thumb. We can easily express the idea without this specific wording.

site/content/tutorial/02-reactivity/04-updating-arrays-and-objects/text.md

@@ -31,7 +31,7 @@ function addNumber() {
 }
 ```
 
-A simple rule of thumb: the name of the updated variable must appear on the left hand side of the assignment. For example this...
+A simple rule: the name of the updated variable must appear on the left hand side of the assignment. For example this...
 
 ```js
 const foo = obj.foo;