[css-text] Fix heading.

Author: fantasai

css-text/Overview.bs

@@ -1505,7 +1505,7 @@ Cursive Scripts</h4>
     the letter-combinations involved, location within the word, and location of the word within the line.
 
 <h4 id="justify-algos">
-Possible Algorithms</h4>
+Minimum Requirements for ''text-justify/auto'' Justification</h4>
 
   <p>For <a value for=text-justify>auto</a> justification, this specification does not define
   what all of the <a>justification opportunities</a> are,