(launch) fix describe of PathJoinSubstitution (#771)

Signed-off-by: Matthijs van der Burgh <[email protected]>

launch/launch/substitutions/path_join_substitution.py

@@ -38,7 +38,7 @@ def substitutions(self) -> Iterable[Substitution]:
 
     def describe(self) -> Text:
         """Return a description of this substitution as a string."""
-        return "LocalVar('{}')".format(' + '.join([s.describe() for s in self.substitutions]))
+        return f"PathJoin('{' + '.join([s.describe() for s in self.substitutions])}')"
 
     def perform(self, context: LaunchContext) -> Text:
         """Perform the substitution by retrieving the local variable."""