site: rephrase to clarify that actions can have just one parameter.

Closes #5173

Author: mindrones

site/content/docs/02-template-syntax.md

@@ -773,7 +773,7 @@ Actions are functions that are called when an element is created. They can retur
 
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-An action can have parameters. If the returned value has an `update` method, it will be called whenever those parameters change, immediately after Svelte has applied updates to the markup.
+An action can have a parameter. If the returned value has an `update` method, it will be called whenever that parameter changes, immediately after Svelte has applied updates to the markup.
 
 > Don't worry about the fact that we're redeclaring the `foo` function for every component instance — Svelte will hoist any functions that don't depend on local state out of the component definition.