Remove outdated alias from help show output.

Fixes #806

src/Command/ShowCommand.php

@@ -46,7 +46,7 @@ protected function configure()
 Show the code for an object, class, constant, method or property, or the context
 of the last exception.
 
-<return>cat --ex</return> defaults to showing the lines surrounding the location of the last
+<return>show --ex</return> defaults to showing the lines surrounding the location of the last
 exception. Invoking it more than once travels up the exception's stack trace,
 and providing a number shows the context of the given index of the trace.