Fix duplicated word in manual

Closes codemirror#6823

Author: marijnh

doc/manual.html

@@ -1828,7 +1828,7 @@ <h3 id="api_decoration">Widget, gutter, and decoration methods</h3>
       <dt id="addLineClass"><code><strong>doc.addLineClass</strong>(line: integer|LineHandle, where: string, class: string) → LineHandle</code></dt>
       <dd>Set a CSS class name for the given line. <code>line</code>
       can be a number or a line handle. <code>where</code> determines
-      to which element this class should be applied, can can be one
+      to which element this class should be applied, can be one
       of <code>"text"</code> (the text element, which lies in front of
       the selection), <code>"background"</code> (a background element
       that will be behind the selection), <code>"gutter"</code> (the