offload: Update help string.

* guix/scripts/offload.scm (guix-offload): Update help string.

guix/scripts/offload.scm

@@ -779,7 +779,8 @@ (define not-coma
     (("--version")
      (show-version-and-exit "guix offload"))
     (("--help")
-     (format #t (G_ "Usage: guix offload SYSTEM PRINT-BUILD-TRACE
+     (format #t (G_ "Usage: guix offload SYSTEM MAX-SILENT-TIME \
+PRINT-BUILD-TRACE? BUILD-TIMEOUT
 Process build offload requests written on the standard input, possibly
 offloading builds to the machines listed in '~a'.~%")
              %machine-file)