Replace "restart shell" documentation "exec zsh" (#8411)

Running `source ~/.zshrc` can have side effects, it's better to reload the whole 
zsh session with `exec zsh`.

plugins/catimg/README.md

@@ -17,10 +17,10 @@ Plugin for displaying images on the terminal using the the `catimg.sh` script pr
    )
    ```
 
-2. Reload the source file or restart your Terminal session:
+2. Restart the shell or restart your Terminal session:
 
    ```console
-   $ source ~/.zshrc
+   $ exec zsh
    $
    ```