Fix potential issue in dsf alias with spaces

If there are no quotes around the arguments the dsf alias won't work
with spaces when used in bash. (Does seem to work in zsh.)

Goes to issue so-fancy#328 / PR so-fancy#358

README.md

@@ -31,7 +31,7 @@ diff -u file_a file_b | diff-so-fancy
 You could, of course, create a function for it too. In your `~/.bashrc` or `~/.zshrcz` for example:
 
 ```shell
-function dsf() { diff -u $1 $2 | diff-so-fancy; }
+function dsf() { diff -u "$1" "$2" | diff-so-fancy; }
 ```
 
 And than you can just type `dsf file_a file_b` to get nice looking diff output.