Rename formats-util.h to format-util.h

We don't have plural in the name of any other -util files and this
inconsistency trips me up every time I try to type this file name
from memory. "formats-util" is even hard to pronounce.

src/systemctl/systemctl.c

@@ -50,7 +50,7 @@
 #include "exit-status.h"
 #include "fd-util.h"
 #include "fileio.h"
-#include "formats-util.h"
+#include "format-util.h"
 #include "fs-util.h"
 #include "glob-util.h"
 #include "hostname-util.h"