v.edit: -t => -b in the manual (#471)

vector/v.edit/v.edit.html

@@ -31,7 +31,7 @@ covered by two bounding boxes (center at 599505,4921010 and
 
 <h2>NOTES</h2>
 
-If no vector features are selected or the flag <b>-t</b> is
+If no vector features are selected or the flag <b>-b</b> is
 used, topology is not build at the end.
 
 <h2>USAGE</h2>