Corrects the path of keysymdef.h.

Index: etc/nsxiv.1
--- etc/nsxiv.1.orig
+++ etc/nsxiv.1
@@ -559,7 +559,7 @@ By default nsxiv(1) will send one image per-line to st
 
 The key combo argument has the following form: "[C-][M-][S-]KEY",
 where C/M/S indicate Ctrl/Meta(Alt)/Shift modifier states and KEY is the X
-keysym as listed in /usr/include/X11/keysymdef.h without the "XK_" prefix.
+keysym as listed in ${X11BASE}/include/X11/keysymdef.h without the "XK_" prefix.
 If KEY has an uppercase equivalent, S-KEY is resolved into it. For instance,
 K replaces S-k and Scedilla replaces S-scedilla, but S-Delete is sent as-is.
 
