Fix install instructions in README because of package name

[patrickmineault]

Copying and pasting the pip3 install line, I got the following error:

Discarding git+https://github.com/dtcooper/python-fitparse#egg=python-fitparse: Requested fitparse from git+https://github.com/dtcooper/python-fitparse#egg=python-fitparse has inconsistent name: filename has 'python-fitparse', but metadata has 'fitparse'
ERROR: Could not find a version that satisfies the requirement python-fitparse (unavailable) (from versions: none)
ERROR: No matching distribution found for python-fitparse (unavailable)

Switching the egg name fixed it.