fix #H2709 - exercise_steed() for riding skill

Code appears to intend that riding for 100 turns be treated like a
successful weapon hit as far as skill training goes, but it was
actually requiring 101 turns each time.  It's conceivable that that
was intentional, but unlikely.

doc/fixes36.2

@@ -299,6 +299,7 @@ identifying or forgetting gem types now adjusts prices for gems already on
 	shopping bill
 when fire converts an ice location into a water location, dunk any monster on
 	that spot immediately instead of waiting until its next move
+training riding skill had an off-by-one bug when counting turns riding
 
 
 Fixes to Post-3.6.1 Problems that Were Exposed Via git Repository

src/steed.c

@@ -1,4 +1,4 @@
-/* NetHack 3.6	steed.c	$NHDT-Date: 1544666049 2018/12/13 01:54:09 $  $NHDT-Branch: NetHack-3.6.2-beta01 $:$NHDT-Revision: 1.61 $ */
+/* NetHack 3.6	steed.c	$NHDT-Date: 1545441042 2018/12/22 01:10:42 $  $NHDT-Branch: NetHack-3.6.2-beta01 $:$NHDT-Revision: 1.62 $ */
 /* Copyright (c) Kevin Hugo, 1998-1999. */
 /* NetHack may be freely redistributed.  See license for details. */
 
@@ -366,7 +366,7 @@ exercise_steed()
         return;
 
     /* It takes many turns of riding to exercise skill */
-    if (u.urideturns++ >= 100) {
+    if (++u.urideturns >= 100) {
         u.urideturns = 0;
         use_skill(P_RIDING, 1);
     }