style: avoid using number in prose

Author: marcoscaceres

index.html

@@ -523,8 +523,8 @@ <h3>
           <a>BeforeInstallPromptEvent.prompt</a>() method).
         </p>
         <p>
-          The <a>BeforeInstallPromptEvent</a> has three internal slots, which
-          are set when the event is <a data-lt=
+          The <a>BeforeInstallPromptEvent</a> has the following internal slots,
+          which are set when the event is <a data-lt=
           "construct a BeforeInstallPromptEvent">constructed</a>:
         </p>
         <dl>