Fix typo: USERNAME ->USER_NAME (#5)

README.md

@@ -22,7 +22,7 @@ REPO_NAME_CONTAINS=string-to-match-repositories-with # Default: ""
 CRONTAB_SCHEDULE=crontab-schedule-to-get-data-from-github # Default: "0 * * * *"
 ```
 
-**_NOTE:_** You also can use your username if you don't belong to any organization. In this case, use "USERNAME" instead of "ORG_NAME".
+**_NOTE:_** You also can use your username if you don't belong to any organization. In this case, use "USER_NAME" instead of "ORG_NAME".
 
 Run the image:
 ```
@@ -69,4 +69,4 @@ Removing github-traffic_grafana_1    ... done
 Removing github-traffic_prometheus_1 ... done
 Removing github-traffic_traffic_1    ... done
 Removing network github-traffic_default
-```
+```