Bugfix, fixed a broken command-line example

The last example in the table contradicted the previous example. Surely
it's purpose was to showcase the @ behaviour to invert threshold, but the
@ sign was missing.

I also clarified the description to the second last example so it is in same
style as the others.

doc/developer-guidelines.sgml

@@ -307,10 +307,10 @@
 					</row>
 					<row>
 						<entry>check_stuff -c5:6</entry>
-						<entry>The only noncritical range is 5:6</entry>
+						<entry>Critical if "stuff" is outside of 5:6</entry>
 					</row>
 					<row>
-						<entry>check_stuff -c10:20</entry>
+						<entry>check_stuff -c@10:20</entry>
 						<entry>Critical if "stuff" is less than 10 or over 20</entry>
 					</row>
 				</tbody>