typo in conda install command (#466)

Regardless of the typo fix, these instructions do not seem to work.

upon following steps 2, 3, and 4 (`conda install -c usgs-astrogeology autocnet`)
I am unable to succesfully use autocnet. 

`> from autocnet.graph.network import NetworkCandidateGraph`
```
~/anaconda3/envs/autocnet/lib/python3.7/site-packages/setuptools/distutils_patch.py:26: UserWarning: Distutils was imported before Setuptools. This usage is discouraged and may exhibit undesirable behaviors or errors. Please use Setuptools' objects directly or at least import Setuptools first.
  "Distutils was imported before Setuptools. This usage is discouraged "
~/anaconda3/envs/autocnet/lib/python3.7/site-packages/pysis/env.py:31: RuntimeWarning: Could not find isis. Is `ISISROOT` set?
  warnings.warn('Could not find isis. Is `ISISROOT` set?', RuntimeWarning)
```

What is ISISROOT? Is [the isis package](https://github.com/USGS-Astrogeology/ISIS3#installation) a dependency that I need to download and configure?