Update README.rst

the gtd.py has no matching in pip. instead, it has successfully installed using pip3

executed
```
pip install -U gtd.py
```

result
```
Could not find a version that satisfies the requirement gtd.py (from versions: )
No matching distribution found for gtd.py
```

README.rst

@@ -406,7 +406,7 @@ Setup
 
 ::
 
-  $ pip install -u gtd.py
+  $ pip3 install -U gtd.py
   $ gtd onboard
 
 The ``onboard`` command will assist you through the process of getting a Trello API key for use with this program and putting it in the correct file. This will happen automatically if you run a command that requires authentication without having your API keys set.