Indicate that the --inactivity-timeout option takes an argument.

i3lock.1

@@ -28,7 +28,8 @@ i3lock \- improved screen locker
 .IR pointer\|]
 .RB [\|\-u\|]
 .RB [\|\-e\|]
-.RB [\|\-I\|]
+.RB [\|\-I
+.IR timeout\|]
 .RB [\|\-f\|]
 
 .SH DESCRIPTION
@@ -71,7 +72,7 @@ option, DPMS will turn off your screen after 15 minutes of inactivity anyways (i
 you did not disable this in your X server).
 
 .TP
-.B \-I, \-\-inactivity-timeout
+.BI \-I\  seconds \fR,\ \fB\-\-inactivity-timeout= seconds
 Specifies the number of seconds i3lock will wait for another password before
 turning off the monitors, in case you entered a wrong password or canceled by
 pressing Escape. Only makes sense together with \-d. If omitted, the default is

i3lock.c

@@ -858,7 +858,7 @@ int main(int argc, char *argv[]) {
                 break;
             default:
                 errx(EXIT_FAILURE, "Syntax: i3lock [-v] [-n] [-b] [-d] [-c color] [-u] [-p win|default]"
-                                   " [-i image.png] [-t] [-e] [-I] [-f]");
+                                   " [-i image.png] [-t] [-e] [-I timeout] [-f]");
         }
     }