Small bug-fix due to changes to relationship-type

[Atmire-Kristof]

After the recent merge of #461 into master, my local builds were failing.
This is the error I received:

ERROR in src/app/core/data/relationship.service.ts(185,25): error TS2339: Property 'leftLabel' does not exist on type 'RelationshipType'.
    src/app/core/data/relationship.service.ts(187,25): error TS2339: Property 'rightLabel' does not exist on type 'RelationshipType'.

I renamed these leftover properties to their appropriate name (as they seem to have been forgotten in the PR mentioned above).

[benbosman]

I've tested this locally and it worked correctly for me.
The changes seem correct to me as well
This seems to have been forgotten in #461 although building #461 did work initially

[tdonohue]

👍 This looks obvious to me. These various properties have been renamed, so using the old names definitely will not work. Merging this immediately as it seems like an obvious quick fix.