swi-prolog: avoid misleading audit warning

Due to the unnecessary inner quotes, the audit check wouldn't see the
pipe and misleadingly complain:

  swi-prolog:
    * Use `system "echo", "'` instead of `system "echo '`

Formula/swi-prolog.rb

@@ -69,7 +69,7 @@ def install
     # './prepare' prompts the user to build documentation
     # (which requires other modules). '3' is the option
     # to ignore documentation.
-    system "echo '3' | ./prepare" if build.head?
+    system "echo 3 | ./prepare" if build.head?
     system "./configure", *args
     system "make"
     system "make", "install"