fix manual page typo from #846

raster/r.cost/r.cost.html

@@ -112,7 +112,7 @@ can be controlled with the solver option (blue).</i>
 </center>
 
 <p>
-Multiple derections can be solved as in the above example with the 
+Multiple directions can be solved as in the above example with the 
 following steps:
 <ol>
     <li>Create multiple directions with <b>r.cost</b>/<b>r.walk</b>