Fixed the type of loopback in the docs.

doc/Control

@@ -127,6 +127,6 @@ slowest : [ |<a_1>|, |<a_2>|, ..., |<a_n>| ] -> |< a_1+a_2+...+a_n >|
 Loops
 -----
 
-loopback : (a |->| 'stop'+{'ok', a}) -> |<a>|
+loopback : (a |->| {'stop', b}+{'loop', a}) -> (a |->| b)
 
 loopback terminates when the operation returns 'stop'.