[Minor]: Replace 'train' with 'guard' to make sense.

[Benjamin Jacob Reji]

Link to page(s) with issue:

• https://planning.wiki/ref/pddl22/domain#derived-predicates (last line)

Quote of current error:
"The example above specifies that a train is only usable if it has a train and a driver."
Proposed correction:
Replace 'train' with 'guard' to make sense.
"The example above specifies that a train is only usable if it has a guard and a driver."

[Benjamin Jacob Reji]

#83 Should fix this typo/Rephrasing