[css-backgrounds-3] typo: perimenter -> perimeter

Author: nschonni

css-backgrounds-3/Overview.bs

@@ -2806,7 +2806,7 @@ Shadow Shape, Spread, and Knockout</h4>
   The shadow is drawn inside the padding edge only:
   it is clipped outside the padding box of the element.
 
-  <p>If a <i>spread distance</i> is defined, the shadow perimenter defined above
+  <p>If a <i>spread distance</i> is defined, the shadow perimeter defined above
   is expanded outward (for <i>outer box-shadows</i>) or contracted inward (for <i>inner box-shadows</i>)
   by outsetting (insetting, for inner shadows) the shadow's straight edges by the <i>spread distance</i>
   (and flooring the resulting width/height at zero).