README: fix installation urls

pip3 install says:
WARNING: Discarding git+https://github.com/systemd/python-systemd.git#egg=systemd. Requested systemd-python from git+https://github.com/systemd/python-systemd.git#egg=systemd has inconsistent name: filename has 'systemd', but metadata has
 'systemd-python'

Closes #97.

Author: keszybz

README.md

@@ -35,12 +35,12 @@ To build from source
 On CentOS, RHEL, and Fedora with Python 2:
 
     dnf install git python-pip gcc python-devel systemd-devel
-    pip install git+https://github.com/systemd/python-systemd.git#egg=systemd
+    pip install git+https://github.com/systemd/python-systemd.git#egg=systemd-python
 
 On Fedora with Python 3:
 
     dnf install git python3-pip gcc python3-devel systemd-devel
-    pip3 install git+https://github.com/systemd/python-systemd.git#egg=systemd
+    pip3 install git+https://github.com/systemd/python-systemd.git#egg=systemd-python
 
 On Debian or Ubuntu with Python 2: