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/notify/notify.c

@@ -27,7 +27,7 @@
 
 #include "alloc-util.h"
 #include "env-util.h"
-#include "formats-util.h"
+#include "format-util.h"
 #include "log.h"
 #include "parse-util.h"
 #include "string-util.h"