Fix some typographical errors in comments

Fixes: #153
Applied from: #153

src/engine_base.h

@@ -30,7 +30,7 @@ struct Engine : EnginePool::PoolItem<&_engine_pool> {
 	uint16 reliability_final;   ///< Final reliability of the engine.
 	uint16 duration_phase_1;    ///< First reliability phase in months, increasing reliability from #reliability_start to #reliability_max.
 	uint16 duration_phase_2;    ///< Second reliability phase in months, keeping #reliability_max.
-	uint16 duration_phase_3;    ///< Third reliability phase on months, decaying to #reliability_final.
+	uint16 duration_phase_3;    ///< Third reliability phase in months, decaying to #reliability_final.
 	byte flags;                 ///< Flags of the engine. @see EngineFlags
 	CompanyMask preview_asked;  ///< Bit for each company which has already been offered a preview.
 	CompanyID preview_company;  ///< Company which is currently being offered a preview \c INVALID_COMPANY means no company.

src/script/api/script_order.hpp

@@ -29,7 +29,7 @@ public:
 		/** No more space for orders */
 		ERR_ORDER_TOO_MANY,                                  // [STR_ERROR_NO_MORE_SPACE_FOR_ORDERS]
 
-		/** Destination of new order is to far away from the previous order */
+		/** Destination of new order is too far away from the previous order */
 		ERR_ORDER_TOO_FAR_AWAY_FROM_PREVIOUS_DESTINATION,    // [STR_ERROR_TOO_FAR_FROM_PREVIOUS_DESTINATION]
 
 		/** Aircraft has not enough range to copy/share orders. */

src/station_base.h

@@ -72,7 +72,7 @@ public:
 	/**
 	 * Invalid constructor. This can't be called as a FlowStat must not be
 	 * empty. However, the constructor must be defined and reachable for
-	 * FlwoStat to be used in a std::map.
+	 * FlowStat to be used in a std::map.
 	 */
 	inline FlowStat() {NOT_REACHED();}