[css-text] Resolve ambiguity

Author: frivoal

css-text/Overview.bs

@@ -752,7 +752,7 @@ Languages and Typesetting</h4>
       <li>A sequence of <a>collapsible</a> spaces at the end of a line is removed.
       <li>If spaces or tabs at the end of a line are non-collapsible
          but have 'white-space' set to ''pre-wrap''
-         the UA may either <a>hang</a> the <a>white space</a>
+         the UA must either <a>hang</a> the <a>white space</a>
          or visually collapse their character advance widths
          such that they don't take up space in the line.