docs - add missing <td> column

all other functions in the built-in functions list have three columns.
the last one only has two.
add a blank column so it matches, and the html validates

Author: culix-7

docs/manual/index.html

@@ -2780,6 +2780,7 @@ <h4 id="built-ins">Built-in Functions</h4>
 			<tr>
 				<td><code class="65c816_asar">realbase()</code></td>
 				<td>Returns the current address in the ROM being written to. This is not the same as the value of a nearby label when the <code>base</code> command is active: it returns the actual address the code will end up at.</td>
+				<td><pre><code></code></pre></td>
 			</tr>
 		</table><br/>
 		All built-in functions can be overridden by user-defined functions. Prepending an underscore (for example: <code class="65c816_asar">_read1()</code>) leads to the original function, which can be used to make an overridden function call its original function.