[README] Fix bogus command to add user to group (#255)

I typed 'useradd' when I meant 'usermod' and worse, without the -a
flag it wipes out your other group memberships.

Signed-off-by: Geoff Gustafson <[email protected]>

Author: grgustaf

README.md

@@ -67,7 +67,7 @@ $ source ~/.bashrc
 Add your user to the plugdev group with this command:
 
 ```bash
-$ sudo useradd -G plugdev USERNAME
+$ sudo usermod -a -G plugdev USERNAME
 ```
 
 #### Add udev rules