Remove old mention of p4d2 2017 directory (#620)

* Support negative-valued action parameters
by converting them in the Python helper code into a positive value
whose bit pattern is the 2's complement representation of the negative
value.

* Remove mention of old directory P4D2_2017_Fall
that has long since been removed from the main branch of this repo.

Author: jafingerhut

exercises/p4runtime/README.md

@@ -175,7 +175,7 @@ make clean
 #### Running the reference solution
 
 To run the reference solution, you should run the following command from the
-`~/tutorials/P4D2_2017_Fall/exercises/p4runtime` directory:
+`~/tutorials/exercises/p4runtime` directory:
 ```bash
 solution/mycontroller.py
 ```