[manual] Fix inaccuate claim that getModeAt exists on docs

Issue #3214

doc/manual.html

@@ -1849,7 +1849,7 @@ editor.setOption("extraKeys", {
       the mode specification, rather than the resolved, instantiated
       <a href="#defineMode">mode object</a>.</dd>
 
-      <dt id="getModeAt"><code><strong>doc.getModeAt</strong>(pos: {line, ch}) → object</code></dt>
+      <dt id="getModeAt"><code><strong>cm.getModeAt</strong>(pos: {line, ch}) → object</code></dt>
       <dd>Gets the inner mode at a given position. This will return
       the same as <a href="#getMode"><code>getMode</code></a> for
       simple modes, but will return an inner mode for nesting modes