Change Content Blockers to Extensions (#268)

Currently, the splash page says to go to Safari's settings and enable the extension in the `Content Blockers` tab. But that's not quite right, at least with the latest version of Safari. So I changed that to `Extensions`.

docs/index.html

@@ -583,7 +583,7 @@ <h2>Fast</h2>
     <div class="usp">
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       <h2>Simple</h2>
-      <p>It's as easy as downloading the app and enabling it in <em>Safari</em> settings ⭢ <em>Content Blockers</em>. No configuration or maintenance needed.</p>
+      <p>It's as easy as downloading the app and enabling it in <em>Safari</em> settings ⭢ <em>Extensions</em>. No configuration or maintenance needed.</p>
     </div>
     <div class="usp">
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