Docs: Rename slot let section (#6018)

The title of this section was a bit confusing, since `<slot let:name={value} />` is not actually the correct syntax

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

@@ -1376,7 +1376,7 @@ Note that explicitly passing in an empty named slot will add that slot's name to
 </Card>
 ```
 
-#### [`<slot let:`*name*`={`*value*`}>`](slot_let)
+#### [`<slot key={`*value*`}>`](slot_let)
 
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