docs: remove obsolete Ctrl+Z from the cheatsheet; mention Alt+X instead

This should have been part of commit d0c2c1e6 from seven weeks ago.

Reported-by: Tasos Papastylianou <tpapastylianou@hotmail.com>

doc/cheatsheet.html

@@ -118,11 +118,11 @@
 <tr><td>Alt+A</td><td>Turn the mark on/off</td></tr>
 <tr><td>Tab</td><td>Indent marked region</td></tr>
 <tr><td>Shift+Tab &nbsp;&nbsp;</td><td>Unindent marked region</td></tr>
+<tr><td>Alt+V</td><td>Enter next keystroke verbatim</td></tr>
 <tr><td>Alt+N</td><td>Turn line numbers on/off</td></tr>
 <tr><td>Alt+P</td><td>Turn visible whitespace on/off</td></tr>
-<tr><td>Alt+V</td><td>Enter next keystroke verbatim</td></tr>
+<tr><td>Alt+X</td><td>Hide or unhide the help lines</td></tr>
 <tr><td>Ctrl+L</td><td>Refresh the screen</td></tr>
-<tr><td>Ctrl+Z</td><td>Suspend nano</td></tr>
 </tbody></table>
 <br>