Fix some a/an related typos

Author: Psychpsyo

css-images-3/Overview.bs

@@ -1191,8 +1191,8 @@ Object-Sizing Terminology {#sizing-terms}
 			CSS does not define how the natural dimensions are found in general.
 
 			Raster images are an example of an [=object=] with all three natural dimensions.
-			SVG images designed to scale might have only an [=natural aspect ratio=];
-			SVG images can also be created with only an [=natural width=] or [=natural height|height=].
+			SVG images designed to scale might have only a [=natural aspect ratio=];
+			SVG images can also be created with only a [=natural width=] or [=natural height|height=].
 			CSS gradients, defined in this specification,
 			are an example of an object with no [=natural dimensions=] at all.
 			Another example of this is embedded documents,
@@ -2498,4 +2498,4 @@ Changes Since the <a href="https://www.w3.org/TR/2012/CR-css3-images-20120417/">
 	<!-- CSS Values 4 test, empty url, on background-image -->
 	<wpt hidden>
 		empty-background-image.html
-	</wpt>
+	</wpt>