Correct article "an" for @font-face

Author: tomayac

css-fonts-3/Overview-wip.bs

@@ -1414,7 +1414,7 @@ name must not be used.
 }
 </pre>
 
-Just as a ''@font-face'' rule specifies the characteristics of a single font
+Just as an ''@font-face'' rule specifies the characteristics of a single font
 within a family, the unique name used with ''local()'' 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
@@ -3677,7 +3677,7 @@ the following extensions to the CSS Object Model.
 
 <h3 id="om-fontface">The {{CSSFontFaceRule}} interface</h3>
 
-The <dfn interface>CSSFontFaceRule</dfn> interface represents a ''@font-face'' rule.
+The <dfn interface>CSSFontFaceRule</dfn> interface represents an ''@font-face'' rule.
 
 <pre class="idl">
 interface CSSFontFaceRule : CSSRule {