um: time-travel: fix time-travel-start option

We need to have the = as part of the option so that the
value can be parsed properly. Also document that it must
be given in nanoseconds, not seconds.

Fixes: 06503870 ("um: Support time travel mode")
Link: https://patch.msgid.link/20240417102744.14b9a9d4eba0.Ib22e9136513126b2099d932650f55f193120cd97@changeidSigned-off-by: Johannes Berg <johannes.berg@intel.com>

arch/um/kernel/time.c

@@ -875,9 +875,9 @@ static int setup_time_travel_start(char *str)
 	return 1;
 }
 
-__setup("time-travel-start", setup_time_travel_start);
+__setup("time-travel-start=", setup_time_travel_start);
 __uml_help(setup_time_travel_start,
-"time-travel-start=<seconds>\n"
+"time-travel-start=<nanoseconds>\n"
 "Configure the UML instance's wall clock to start at this value rather than\n"
 "the host's wall clock at the time of UML boot.\n");
 #endif