Fix Package Name typo in pip install instructions (#609)

Change package name in pip install instructions to match pypi with - instead of _

Signed-off-by: Norman Gehrsitz <gehrsitz.norman@student.dhbw-kahlsruhe.de>

Author: ngehrsitz

README.md

@@ -6,7 +6,7 @@ The official Python 2 and 3 client for [Prometheus](http://prometheus.io).
 
 **One**: Install the client:
 ```
-pip install prometheus_client
+pip install prometheus-client
 ```
 
 **Two**: Paste the following into a Python interpreter: