readme: keep lower case

Signed-off-by: Marc-Aurèle Brothier <m@brothier.org>

Author: marcaurele

README.md

@@ -502,7 +502,7 @@ There's several steps to getting this working:
 
 **1. Gunicorn deployment**:
 
-The `PROMETHEUS_MULTIPROC_DIR` environment variable must be set to a directory
+The `prometheus_multiproc_dir` environment variable must be set to a directory
 that the client library can use for metrics. This directory must be wiped
 between Gunicorn runs (before startup is recommended).