small grammar fix of dwm.1

dwm.1

@@ -71,10 +71,10 @@ Grow master area (tiling mode only).
 Shrink master area (tiling mode only).
 .TP
 .B Mod1-i
-Increase windows of master area (tiling mode only).
+Increase the number of windows in the master area (tiling mode only).
 .TP
 .B Mod1-d
-Decrease windows of master area (tiling mode only).
+Decrease the number of windows in the master area (tiling mode only).
 .TP
 .B Mod1-Shift-[1..n]
 Apply