fetcher: docstring fix

Make the dosctring match the similar argument in download_bytes()

Signed-off-by: Jussi Kukkonen <[email protected]>

tuf/ngclient/fetcher.py

@@ -77,8 +77,7 @@ def download_file(self, url: str, max_length: int) -> Iterator[IO]:
 
         Args:
             url: a URL string that represents the location of the file.
-            max_length: an integer value representing the length of
-                the file or an upper bound.
+            max_length: upper bound of file size in bytes.
 
         Raises:
             exceptions.DownloadError: An error occurred during download.