signature abstract subtype is missing abstract in example

[pmonson711]

https://alloy.readthedocs.io/en/latest/language/signatures.html#abstract

I believe the example should be :

abstract sig Machine {}
sig Broken in Machine {}

sig Server extends Machine {}
sig Client extends Machine {}

To show that you can have Clients or Servers possibly in Broken but no Machines, or Broken Machines.

[hwayne]

Fixed, thanks for catching!