Fix extremely important article use before 'box'

I am pedantic and I can't handle it.

Author: would-code-again

css-paint-api/Overview.bs

@@ -49,11 +49,11 @@ Introduction {#intro}
 The paint stage of CSS is responsible for painting the background, content and highlight of a
 box based on that box's size (as generated by the layout stage) and computed style.
 
-This specification describes an API which allows developers to paint a part of an box in
+This specification describes an API which allows developers to paint a part of a box in
 response to size / computed style changes with an additional <<image>> function.
 
 Note: In a future version of the spec, support may be added for defining the clip, global alpha,
-    filter on a portion of an box (for example on the background layers).
+    filter on a portion of a box (for example on the background layers).
 
 Paint Invalidation {#paint-invalidation}
 ========================================
@@ -67,7 +67,7 @@ Each <<paint()>> function for a box has an associated <dfn>paint valid flag</dfn
 When the size (as determined by layout) of a |box| changes, each <<paint()>> function's <a>paint
 valid flag</a> should be set to <a>paint-invalid</a>.
 
-When the computed style for an |box| changes, the user agent must run the following steps:
+When the computed style for a |box| changes, the user agent must run the following steps:
     1. For each <<paint()>> function on the |box|, perform the following substeps:
         1. Let |paintFunction| be the current <<paint()>> function on the |box|.