Fix javadoc typos.

Author: jmrozanec

src/main/java/com/cronutils/mapper/CronMapper.java

@@ -49,6 +49,7 @@ public class CronMapper {
      *             if null a NullPointerException will be raised
      * @param to - target CronDefinition;
      *             if null a NullPointerException will be raised
+     * @param cronRules - cron rules
      */
     public CronMapper(CronDefinition from, CronDefinition to, Function<Cron, Cron> cronRules){
         Preconditions.checkNotNull(from, "Source CronDefinition must not be null");

src/main/java/com/cronutils/model/time/ExecutionTime.java

@@ -136,7 +136,7 @@ public static ExecutionTime forCron(Cron cron) {
     /**
      * Provide nearest date for next execution.
      * @param date - ZonedDateTime instance. If null, a NullPointerException will be raised.
-     * @return Optional<ZonedDateTime> instance, never null. Contains next execution time or empty.
+     * @return Optional ZonedDateTime instance, never null. Contains next execution time or empty.
      */
     public Optional<ZonedDateTime> nextExecution(ZonedDateTime date) {
         Preconditions.checkNotNull(date);
@@ -443,7 +443,7 @@ public Optional<Duration> timeToNextExecution(ZonedDateTime date){
     /**
      * Provide nearest date for last execution.
      * @param date - ZonedDateTime instance. If null, a NullPointerException will be raised.
-     * @return Optional<ZonedDateTime> instance, never null. Last execution time or empty.
+     * @return Optional ZonedDateTime instance, never null. Last execution time or empty.
      */
     public Optional<ZonedDateTime> lastExecution(ZonedDateTime date){
         Preconditions.checkNotNull(date);

src/main/java/com/cronutils/utils/Preconditions.java

@@ -158,7 +158,7 @@ public static void checkState(boolean expression,
    * Ensures that an object reference passed as a parameter to the calling method is not null.
    *
    * @param reference an object reference
-   * @return the non-null reference that was validated
+   * @return T - the non-null reference that was validated
    * @throws NullPointerException if {@code reference} is null
    */
   public static <T> T checkNotNull(T reference) {