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