Update use-server.mdx Single-flight "actions" s/b "mutations"

The topic Single-flight "mutations" in response-helpers>redirect.mdx also uses `action` and I haven't heard the term Single-flight "actions" used in doc or videos.

- Replace "actions" with "mutations"

Author: klequis

src/routes/solid-start/reference/server/use-server.mdx

@@ -63,7 +63,7 @@ const updateUser = action(async (id, data) => {
 
 When `getUser` or `updateUser` are invoked on the client, an http request will be made to the server, which calls the corresponding server function.
 
-## Single-flight actions
+## Single-flight mutations
 
 In the above example, when the `updateUser` action is called, a redirect is thrown on the server. 
 Solid Start can handle this redirect on the server instead of propagating it to the client.