Another fix for 'apt-get <...> -t' synopsis.

doc/apt-get.8.xml

@@ -48,10 +48,10 @@
 		  <option>-t=</option>
 		  <group choice='req'>
 			  <arg choice='plain'>
-				  target_release_name
+				  <replaceable>target_release_name</replaceable>
 			  </arg>
 			  <arg choice='plain'>
-				  target_release_number_expression
+				  <replaceable>target_release_number_expression</replaceable>
 			  </arg>
 		  </group>
       </arg>