docs: hacking: add missing code element

Wrap the 'g_renew()' call for VIR_SHRINK_N in <code>.
Signed-off-by: NJán Tomko <jtomko@redhat.com>

docs/hacking.html.in

@@ -1056,7 +1056,7 @@ BAD:
             <td>the newly added memory is not zeroed</td></tr>
         <tr><td><code>VIR_EXPAND_N</code></td><td><code>g_renew(var_t, ptr, n)</code></td>
             <td>zero the new memory manually or use an array type</td></tr>
-        <tr><td><code>VIR_SHRINK_N</code></td><td>g_renew(var_t, ptr, n)</td>
+        <tr><td><code>VIR_SHRINK_N</code></td><td><code>g_renew(var_t, ptr, n)</code></td>
             <td></td></tr>
         <tr><td><code>VIR_APPEND_ELEMENT</code></td><td><code>g_array_append_val</code></td>
             <td><code>g_ptr_array_add</code> or <code>g_byte_array_append</code></td></tr>