Editorial: fix typo (#828)

Author: jugglinmike

index.html

@@ -2605,7 +2605,7 @@ <h3>
           [=set=] or failure.
         </p>
         <ol>
-          <li>If [=Type=](|image|["purpose"]) it not String, or
+          <li>If [=Type=](|image|["purpose"]) is not String, or
           |image|["purpose"] consists solely of [=ascii whitespace=], then
           return the [=set=] « "any" ».
           </li>
@@ -2840,7 +2840,7 @@ <h3>
           </li>
           <li>Let <var>value</var> be <var>image.sizes</var>.
           </li>
-          <li>If <a>Type</a>(<var>value</var>) it not String, return
+          <li>If <a>Type</a>(<var>value</var>) is not String, return
           <code>undefined</code>.
           </li>
           <li>Otherwise, parse <var>value</var> as if it was a [[HTML]]
@@ -2890,7 +2890,7 @@ <h3>
         <ol>
           <li>Let <var>value</var> be <var>image</var>["<a>type</a>"].
           </li>
-          <li>If <a>Type</a>(<var>value</var>) it not String, return
+          <li>If <a>Type</a>(<var>value</var>) is not String, return
           <code>undefined</code>.
           </li>
           <li>If <var>value</var> is not a <a>valid MIME type string</a> or the