added Mod1-Tab description to dwm.1

dwm.1

@@ -59,6 +59,9 @@ Start
 .B Mod1\-Return
 Zooms/cycles current window to/from master area (tiled layout only).
 .TP
+.B Mod1\-Tab
+Toggles to the previously selected tags.
+.TP
 .B Mod1\-b
 Shows/hides the status bar.
 .TP