fix: typo in entry-client.mdx (#833)

Author: J-Schoepplenberg

src/routes/solid-start/reference/entrypoints/entry-client.mdx

@@ -12,4 +12,4 @@ mount(() => <StartClient />, document.getElementById("app")!);
 ```
 
 This file is an ideal place to run any client specific code that is needed on startup, such as registering service workers. 
-This is the important if you are performing client-only rendering or using other modes of server-side rendering.
+This is important if you are performing client-only rendering or using other modes of server-side rendering.