[Editorial] Tweak glossary entry to match others

src/epub/text/appendix-1.xhtml

@@ -89,7 +89,7 @@
 						<dfn>The Pack</dfn>
 					</dt>
 					<dd>
-						<p>is a term very often used in a wide sense to include any area of sea-ice, no matter what form it takes or how disposed. The French term is <i xml:lang="fr">banquise de derive</i>.</p>
+						<p>A term very often used in a wide sense to include any area of sea-ice, no matter what form it takes or how disposed. The French term is <i xml:lang="fr">banquise de derive</i>.</p>
 					</dd>
 					<dt>
 						<dfn>Pack-ice</dfn>