Fix mapdl: _log.logger.add_handler (#968)

* mapdl.add_file_handler: fix _log.logger.addHandler

The object `self._log` has no method `addHandler`, it is in `logger.addHandler`.

* correct speeling of logger

Author: beppo-dd

src/ansys/mapdl/core/mapdl.py

@@ -1620,7 +1620,7 @@ def add_file_handler(self, filepath, append=False, level="DEBUG"):
         if isinstance(level, str):
             level = level.upper()
         self._log_filehandler.setLevel(level)
-        self._log.addHandler(self._log_filehandler)
+        self._log.logger.addHandler(self._log_filehandler)
         self._log.info("Added file handler at %s", filepath)
 
     def remove_file_handler(self):