docs: minor addition to onMount usage (sveltejs#445)

* docs: minor addition to onMount usage

* Update content/tutorial/01-svelte/07-lifecycle/01-onmount/README.md

* Update content/tutorial/01-svelte/07-lifecycle/01-onmount/README.md

---------

Co-authored-by: Rich Harris <[email protected]>

Author: Zerotask

content/tutorial/01-svelte/07-lifecycle/01-onmount/README.md

@@ -4,7 +4,7 @@ title: onMount
 
 Every component has a _lifecycle_ that starts when it is created, and ends when it is destroyed. There are a handful of functions that allow you to run code at key moments during that lifecycle. The one you'll use most frequently is `onMount`, which runs after the component is first rendered to the DOM.
 
-In this exercise, we have a `<canvas>` that we'd like to animate, using the `paint` function in `gradient.js`. Begin by importing the function from `svelte`:
+In this exercise, we have a `<canvas>` that we'd like to animate, using the `paint` function in `gradient.js`. Begin by importing the `onMount` function from `svelte`:
 
 ```svelte
 /// file: App.svelte
@@ -14,7 +14,7 @@ In this exercise, we have a `<canvas>` that we'd like to animate, using the `pai
 </script>
 ```
 
-Then, add a function that runs when the component mounts:
+Then, add a callback that runs when the component mounts:
 
 ```svelte
 /// file: App.svelte