Fix syntax errors in qhelp files

cpp/ql/src/Critical/GlobalUseBeforeInit.qhelp

@@ -20,7 +20,9 @@ the initialization code is executed.
 </p>
 </recommendation>
 <example>
+<p>
 In the example below, <code>callCtr</code> is wrongly used before it has been initialized.
+</p>
 <sample src="GlobalUseBeforeInit.cpp" />
 </example>
 

cpp/ql/src/Critical/InconsistentNullnessTesting.qhelp

@@ -20,9 +20,11 @@ important system data (including the interrupt table in some architectures).
 
 </recommendation>
 <example>
+<p>
 This code shows two examples where a pointer is dereferenced. 
 The first example checks that the pointer is not null before dereferencing it. 
 The second example fails to perform a nullness check, leading to a potential vulnerability in the code.
+</p>
 <sample src="InconsistentNullnessTesting.cpp" />
 </example>
 

cpp/ql/src/Critical/OverflowCalculated.qhelp

@@ -13,6 +13,7 @@ zero terminator into consideration.</li>
 <li><code>strcat</code> or <code>strncat</code> that use buffers that are too small to contain the new string.</li>
 </ul>
 </p>
+</overview>
 
 <p>
 The highlighted expression will cause a buffer overflow because the buffer is too small to contain