Correct article "an" for @font-face

Author: tomayac

css-fonts-3/Fonts.src.html

@@ -1703,7 +1703,7 @@ <h3 id="src-desc">Font reference: the <a href="#descdef-src">src</a> descriptor<
 }
 </pre>
 
-<p>Just as a <code>@font-face</code> rule specifies the characteristics of a single font
+<p>Just as an <code>@font-face</code> rule specifies the characteristics of a single font
 within a family, the unique name used with <code>local()</code> specifies a single
 font, not an entire font family. Defined in terms of
 OpenType font data, the Postscript name is found in the font's
@@ -3840,7 +3840,7 @@ <h2 id="object-model">Object Model</h2>
 
 <h3 id="om-fontface">The <code>CSSFontFaceRule</code> interface</h3>
 
-<p>The <dfn>CSSFontFaceRule</dfn> interface represents a <code>@font-face</code> rule.</p>
+<p>The <dfn>CSSFontFaceRule</dfn> interface represents an <code>@font-face</code> rule.</p>
 
 <pre class="idl">
 interface CSSFontFaceRule : CSSRule {