Fix documentation reference to nonexistant package (#105)

Trying to install python-pip and python-dev on Ubuntu 20.04 results in "Package python-pip is not available", and "selecting 'python-dev-is-python2' instead of 'python-dev'" respectively. Adding the explicit python3 reference fixes the issue

README.md

@@ -54,7 +54,7 @@ $ cd canarytokens-docker
 ```
 * Install Docker compose (if not already present):
 ```
-$ sudo apt-get install python-pip python-dev
+$ sudo apt-get install python3-pip python3-dev
 $ sudo pip install -U docker-compose
 #if this breaks with PyYAML errors, install the libyaml development package
 # sudo apt-get install libyaml-dev