Fix parameter docs for FS.settext (#190)

Match with the docs for `FS.setbytes` because they're different versions
of essentially the same thing: setX to a file, so `path` is exactly the same
and `contents` is adapted for this function.

fs/base.py

@@ -1368,7 +1368,8 @@ def settext(self,
         """Create or replace a file with text.
 
         Arguments:
-            contents (str): A path on the filesystem.
+            path (str): Destination path on the filesystem.
+            contents (str): Text to be written.
             encoding (str, optional): Encoding of destination file
                 (defaults to ``'ut-8'``).
             errors (str, optional): How encoding errors should be treated