spelling: explaining

Signed-off-by: Josh Soref <2119212+jsoref@users.noreply.github.com>

Author: jsoref

javascript/ql/src/Statements/UselessComparisonTest.qhelp

@@ -17,7 +17,7 @@ longer needed, remove it.
 </p>
 
 <p>
-If the check is needed to guard against <code>NaN</code> values, insert a comment explaning the possibility of <code>NaN</code>.
+If the check is needed to guard against <code>NaN</code> values, insert a comment explaining the possibility of <code>NaN</code>.
 </p>
 
 </recommendation>