some spelling corrections

The pull request included some changes that were neither accidental nor
unintentional, so only a subset of the changes from pull request #869
submitted by klorpa were manually applied.

behaviour  -> behavior
speach     -> speech
knowlege   -> knowledge
incrments  -> increments
stethscope -> stethoscope
staiway    -> stairway
arifact    -> artifact
extracing  -> extracting

The uses of "iff" were left alone.

Close #869

src/apply.c

@@ -1061,7 +1061,7 @@ use_mirror(struct obj *obj)
         return ECMD_TIME;
 
     /* couldsee(mtmp->mx, mtmp->my) is implied by the fact that bhit()
-       targetted it, so we can ignore possibility of X-ray vision */
+       targeted it, so we can ignore possibility of X-ray vision */
     vis = canseemon(mtmp);
     /* ways to directly see monster (excludes X-ray vision, telepathy,
        extended detection, type-specific warning) */
@@ -3824,7 +3824,7 @@ do_break_wand(struct obj *obj)
             continue;
         } else if (x != u.ux || y != u.uy) {
             /*
-             * Wand breakage is targetting a square adjacent to the hero,
+             * Wand breakage is targeting a square adjacent to the hero,
              * which might contain a monster or a pile of objects or both.
              * Handle objects last; avoids having undead turning raise an
              * undead's corpse and then attack resulting undead monster.
@@ -3843,7 +3843,7 @@ do_break_wand(struct obj *obj)
             }
         } else {
             /*
-             * Wand breakage is targetting the hero.  Using xdir[]+ydir[]
+             * Wand breakage is targeting the hero.  Using xdir[]+ydir[]
              * deltas for location selection causes this case to happen
              * after all the surrounding squares have been handled.
              * Process objects first, in case damage is fatal and leaves