tool_help: clarify --max-time unit of time is seconds

Before:
 -m, --max-time <time> Maximum time allowed for the transfer

After:
 -m, --max-time <seconds> Maximum time allowed for the transfer

docs/cmdline-opts/max-time.d

@@ -1,6 +1,6 @@
 Long: max-time
 Short: m
-Arg: <time>
+Arg: <seconds>
 Help: Maximum time allowed for the transfer
 See-also: connect-timeout
 ---

src/tool_help.c

@@ -228,7 +228,7 @@ static const struct helptxt helptext[] = {
    "Maximum file size to download"},
   {"    --max-redirs <num>",
    "Maximum number of redirects allowed"},
-  {"-m, --max-time <time>",
+  {"-m, --max-time <seconds>",
    "Maximum time allowed for the transfer"},
   {"    --metalink",
    "Process given URLs as metalink XML file"},