fix typo

Author: fantasai

css3-background/Overview.src.html

@@ -2666,7 +2666,7 @@ <h2>Acknowledgments</h2>
 
 <p>Thanks to Ben Stucki for defining what happens with rounded corners
 if the two adjoining borders are of unequal thickness or one of them
-is zer0.
+is zero.
 
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