Update [Part 2 / Transitions / Key blocks]

Added reference to the value "i" to explain, that the key block rebuilds
when the following expression (after #key) changes, not just
"an expression". This should provide more clarity.

Author: Benkex

content/tutorial/02-advanced-svelte/02-transitions/08-key-blocks/README.md

@@ -2,9 +2,9 @@
 title: Key blocks
 ---
 
-Key blocks destroy and recreate their contents when the value of an expression changes. This is useful if you want an element to play its transition whenever a value changes instead of only when the element enters or leaves the DOM.
+Key blocks destroy and recreate their contents when the value of the following expression (in the example: `i`) changes. This is useful if you want an element to play its transition whenever a value changes instead of only when the element enters or leaves the DOM.
 
-Here, for example, we'd like to play the `typewriter` transition from `transition.js` whenever the loading message changes. Wrap the `<p>` element in a key block:
+Here, for example, we'd like to play the `typewriter` transition from `transition.js` whenever the loading message, i.e. `i` changes. Wrap the `<p>` element in a key block:
 
 ```svelte
 /// file: App.svelte