message: [docs] fix typos in mainloop.hpp
diff: /**
  * @file
  - * This file includes definitions for NCP service.
  + * This file includes definitions for mainloop processor.
  */
  #ifndef OTBR_COMMON_OTBR_MAINLOOP_HPP_
  namespace otbr {
  /**
  - * This type defines the mainloop context that contains context data
  - * for running a mainloop.
  + * This type defines the context data for running a mainloop.
  *
  */
  using MainloopContext = otSysMainloopContext;
  /**
  * This abstract class defines the interface of a mainloop processor
  - * which add fds to the mainloop context and handle fd events.
  + * which adds fds to the mainloop context and handles fds events.
  *
  */
  class MainloopProcessor
  