Minor correction to qos readme (#624)

* 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.

* Make qos/README.md text consistent with the P4 solution

---------

Co-authored-by: jafinger <jafinger@cisco.com>

Author: jafingerhut

exercises/qos/README.md

@@ -77,7 +77,7 @@ missing piece.
 
 First we have to change the ipv4_t header by splitting the TOS field
 into DiffServ and ECN fields.  Remember to update the checksum block
-accordingly.  Then, in the egress control block we must compare the
+accordingly.  Then, in the ingress control block we must compare the
 protocol in IP header with IP protocols. Based on the traffic classes
 and priority, the `diffserv` flag will be set.
 
@@ -178,4 +178,4 @@ The documentation for P4_16 and P4Runtime is available [here](https://p4.org/spe
 
 All excercises in this repository use the v1model architecture, the documentation for which is available at:
 1. The BMv2 Simple Switch target document accessible [here](https://github.com/p4lang/behavioral-model/blob/master/docs/simple_switch.md) talks mainly about the v1model architecture.
-2. The include file `v1model.p4` has extensive comments and can be accessed [here](https://github.com/p4lang/p4c/blob/master/p4include/v1model.p4).
+2. The include file `v1model.p4` has extensive comments and can be accessed [here](https://github.com/p4lang/p4c/blob/master/p4include/v1model.p4).