Spell boundaries correctly

Closes #790.

Author: shs96c

webdriver-spec.html

@@ -8813,7 +8813,7 @@ <h2>Element Displayedness</h2>
  to ascertain an <a>element</a>'s visibility
  is based on crude approximations about its nature and relationship in the tree.
  An <a>element</a> is in general to be considered visible
- if any part of it is drawn on the canvas within the bounderies of the viewport.
+ if any part of it is drawn on the canvas within the boundaries of the viewport.
 
 <p>When asked to
  <dfn data-lt="normalized style pixel float value">normalize style pixel values to floating point</dfn>