readme: document oneliner unattended installation command (#7983)

The original suggestion for an unattended install downloads the installation script to a file, then runs that file with the --unattended argument. The install.sh file would be left behind after the suggested command was run.
This change passes the --unattended argument directly into sh. So, it's a nice one-liner like the default installation script, and it doesn't leave a dangling install.sh script.

README.md

@@ -159,8 +159,7 @@ flag `--unattended` to the `install.sh` script. This will have the effect of not
 the default shell, and also won't run `zsh` when the installation has finished.
 
 ```shell
-curl -Lo install.sh https://raw.githubusercontent.com/robbyrussell/oh-my-zsh/master/tools/install.sh
-sh install.sh --unattended
+sh -c "$(curl -fsSL https://raw.githubusercontent.com/robbyrussell/oh-my-zsh/master/tools/install.sh)" "" --unattended
 ```
 
 #### Installing from a forked repository