Fix typo

Fix a typo where argument had "file" instead of "files".   The plural is what was defined previously in the api and is what Database expects.

python/mspasspy/io/distributed.py

@@ -1366,7 +1366,7 @@ def write_distributed_data(
 
     :param storage_mode: Must be either "gridfs" or "file.  When set to
         "gridfs" the waveform data are stored internally and managed by
-        MongoDB.  If set to "file" the data will be stored in a file system.
+        MongoDB.  If set to "files" the data will be stored in a file system.
         File names are derived from attributes with the tags "dir" and 
         "dfile" in the standard way.   Any datum for which dir or dfile 
         aren't defined will default to the behaviour of the Database 
@@ -1390,7 +1390,7 @@ class method `save_data`.  See the docstring for details but the
         assumes means to store the data in its raw binary form.  The default
         should normally be used for efficiency.  Alternate formats are
         primarily a simple export mechanism.  See the User's manual for
-        more details on data export.  Used only for "file" storage mode.
+        more details on data export.  Used only for "files" storage mode.
     :type file_format: :class:`str`
 
     :param overwrite:  If true gridfs data linked to the original
@@ -1495,7 +1495,7 @@ class method `save_data`.  See the docstring for details but the
         message = "write_distributed_data:  required arg1 (db) must be an instance of mspasspy.db.Database\n"
         message += "Type of arg1 received is {}".format(str(type(db)))
         raise TypeError(message)
-    if storage_mode not in ["file", "gridfs"]:
+    if storage_mode not in ["files", "gridfs"]:
         raise TypeError(
             "write_distributed_data:  Unsupported storage_mode={}".format(storage_mode)
         )