Very minor update on letter casing (#1410)

Update "Add Todo" to "Add todo" so it matches what is in the code.

src/guide/component-slots.md

@@ -25,7 +25,7 @@ Then in the template for `<todo-button>`, you might have:
 </button>
 ```
 
-When the component renders, `<slot></slot>` will be replaced by "Add Todo".
+When the component renders, `<slot></slot>` will be replaced by "Add todo".
 
 ```html
 <!-- rendered HTML -->