Fix qhelp

Author: smowton

java/ql/src/experimental/Security/CWE/CWE-208/TimingAttackAgainstHeader.qhelp

@@ -23,5 +23,6 @@ The following example uses <code>String.equals()</code> method for validating a
 This method implements a non-constant-time algorithm. The example also demonstrates validation using a safe constant-time algorithm.
 </p>
 <sample src="ComparingValueOfSensetiveHeader.java" />
+</example>
 </qhelp>