docs: update README.md (#649)

excercises -> exercises

Signed-off-by: Ikko Eltociear Ashimine <eltociear@gmail.com>

Author: eltociear

README.md

@@ -73,7 +73,7 @@ which contains various examples that you can refer to.
 
 The documentation for P4_16 and P4Runtime is available [here](https://p4.org/specs/)
 
-All excercises in this repository use the v1model architecture, the documentation for which is available at:
+All exercises 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).