docs(api): fix grammar error

Author: Siroshun09

api/src/main/java/com/github/siroshun09/configapi/api/file/PropertiesConfiguration.java

@@ -33,7 +33,7 @@
 import java.util.stream.Collectors;
 
 /**
- * A implementation class that gets value from {@link Properties}.
+ * An implementation class that gets value from {@link Properties}.
  */
 public class PropertiesConfiguration extends AbstractFileConfiguration {
 

api/src/main/java/com/github/siroshun09/configapi/api/serializer/Serializer.java

@@ -39,7 +39,7 @@ public interface Serializer<T, S> {
      * Deserializes the value.
      *
      * @param source the value to deserialize
-     * @return the deserialized value or {@code null} if could not deserialize
+     * @return the deserialized value or {@code null} if the value could not be deserialized
      */
     @Nullable T deserialize(@NotNull Object source);
 }