[css-page-3][editorial] Fixed grammatical mistake (#9679)

Author: SebastianZ

css-page-3/Overview.bs

@@ -711,7 +711,7 @@ where the declaration blocks are intentionally left blank.
 
 Declarations in page and margin contexts <a href="https://www.w3.org/TR/CSS2/cascade.html">cascade</a> just like declarations in style rule for elements.
 
-The <dfn>specificity</dfn> of page a selector is computed in a manner
+The <dfn>specificity</dfn> of a page selector is computed in a manner
 analogous to the computations defined in
 the <a href="https://www.w3.org/TR/CSS2/selector.html">Selectors</a> module: