[css-align-3] Fix typo

Author: fantasai

css-align-3/Overview.bs

@@ -2241,7 +2241,7 @@ Aligning Boxes by Baseline</h3>
 
 
 <h2 id="staticpos-rect">
-ppendix A: Static Position Terminology</h2>
+Appendix A: Static Position Terminology</h2>
 
 	When both <a>inset properties</a> in a given axis are ''left/auto''
 	on an <a href="https://www.w3.org/TR/CSS2/visuren.html#absolutely-positioned">absolutely positioned box</a>,