fix: remove extra dot in docker group add command

Signed-off-by: Aritra Dey <adey01027@gmail.com>

Author: AritraDey-Dev

DEV-INTRO.md

@@ -42,7 +42,7 @@ _Note for Linux (including Windows WSL2) users:_ add your user to the docker gro
 
 ```sh
 # for linux users
-sudo adduser ${USER} docker.
+sudo adduser ${USER} docker
 ```
 
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