Rework the example

[AdrienneCohea]

I am trying graph databases for the first time and ran this example. It compiled and inserted nodes into my neo4j container, but it did not print when Mark was found. I believe this will provide a fix for new users of this crate so they can understand what to expect.

Below are the fixes I identified:

• Query for mark instead of Mark, because neo4j is case-sensitive.
• Spawn query tasks until mark is found, rather than spawning 42 async tasks which may all exit before the nodes are inserted into the graph.
• Join on all tasks so that we always insert nodes into the graph and always query for mark before exiting.