doc: Fix 'pip' example
The imgtool script explicitly calls out python3.  Adjust 'pip' to
'pip3' to reflect this explicit version call as well.
@@ -8,9 +8,9 @@ this script should be preferred to the manual steps described in
This program is written for Python3, and has several dependencies on
Python libraries. These can be installed using 'pip3' manually:
- pip install --user pycrypto
- pip install --user pyasn1
- pip install --user ecdsa
+ pip3 install --user pycrypto
+ pip3 install --user pyasn1
+ pip3 install --user ecdsa
or, on Ubuntu, using the package manager:
