use standard var, thanks @sicarii!

docs/operators/handbook.md

@@ -231,7 +231,7 @@ The requirements are:
 wget -O- https://get.docker.com | bash
 
 # add user to docker group
-usermod -aG docker $YOURUSERNAMEHERE
+usermod -aG docker $USER
 
 # setup swarm
 docker swarm init

docs/operators/tutorial.md

@@ -109,7 +109,7 @@ You need to keep port `:80` and `:443` free on your server for web proxying to y
 wget -O- https://get.docker.com | bash
 
 # add user to docker group
-usermod -aG docker $YOURUSERNAMEHERE
+usermod -aG docker $USER
 
 # setup swarm
 docker swarm init