Update webui.sh

Installation should not be assumed to be located within ~/home directory. User should be expected to install project anywhere and run the startup scripts while in stable-diffusion-webui directory.

See issue #8534

webui.sh

@@ -23,7 +23,7 @@ fi
 # Install directory without trailing slash
 if [[ -z "${install_dir}" ]]
 then
-    install_dir="($pwd)"
+    install_dir="$(pwd)"
 fi
 
 # Name of the subdirectory (defaults to stable-diffusion-webui)