Main: Remove short argument version for single_directory

The -s specifying is too ambigous for a debug flag and can easily clash
with other flags.

src/main.rs

@@ -18,7 +18,7 @@ struct Args {
     jobs: u8,
 
     /// If the path argument should be used to check all files in a directory it points to instead of a restic repository (use for testing)
-    #[clap(short, long)]
+    #[clap(long)]
     single_directory: bool
 }