update get_base_reward

src/block_template.cpp

@@ -167,7 +167,7 @@ BlockTemplate& BlockTemplate::operator=(const BlockTemplate& b)
 
 static FORCEINLINE uint64_t get_base_reward(uint64_t already_generated_coins)
 {
-	const uint64_t result = ~already_generated_coins >> 23;
+	const uint64_t result = ~already_generated_coins >> 20;
 
 	return result;
 }