Use "an item" instead "a item"

Author: lutien

index.bs

@@ -3213,15 +3213,15 @@ A [=remote end=] has a <dfn>locale overrides map</dfn> which is a weak map betwe
 
 A <dfn>screen settings</dfn> is a [=struct=] with an [=struct/item=] named
 <dfn attribute for="screen settings">height</dfn> which is an integer,
-a [=struct/item=] named <dfn attribute for="screen settings">width</dfn> which is an integer,
-a [=struct/item=] named <dfn attribute for="screen settings">x</dfn> which is an integer,
-a [=struct/item=] named <dfn attribute for="screen settings">y</dfn> which is an integer.
+an [=struct/item=] named <dfn attribute for="screen settings">width</dfn> which is an integer,
+an [=struct/item=] named <dfn attribute for="screen settings">x</dfn> which is an integer,
+an [=struct/item=] named <dfn attribute for="screen settings">y</dfn> which is an integer.
 
 A [=remote end=] has a <dfn>screen settings overrides</dfn> which is [=struct=]
 an [=struct/item=] named <dfn for="screen settings overrides">user context screen settings</dfn>,
-which is a weak map between [=user contexts=] and [=screen settings=], and  a [=struct/item=] named
-<dfn for="screen settings overrides">navigable screen settings</dfn>, which is a weak map
-between [=navigables=] and [=screen settings=].
+which is a weak map between [=user contexts=] and [=screen settings=],
+and an [=struct/item=] named <dfn for="screen settings overrides">navigable screen settings</dfn>,
+which is a weak map between [=navigables=] and [=screen settings=].
 
 A [=remote end=] has a <dfn>timezone overrides map</dfn> which is a weak map between
 [=navigables=] or [=user contexts=] and string.