transfer.py: fix docs (#2784)

* transfer.py: fix docs

Currently the docs state that the `use_threads` keyword for `TransferConfig` when set to `False`, means that the operation will run in the main thread. This is incorrect as the operation will run in the _current thread_, which is not necessarily the main thread.
---------

Co-authored-by: Nate Prewitt <nate.prewitt@gmail.com>

boto3/s3/transfer.py

@@ -256,7 +256,7 @@ def __init__(
         :param max_concurrency: The maximum number of threads that will be
             making requests to perform a transfer. If ``use_threads`` is
             set to ``False``, the value provided is ignored as the transfer
-            will only ever use the main thread.
+            will only ever use the current thread.
 
         :param multipart_chunksize: The partition size of each part for a
             multipart transfer.
@@ -281,7 +281,7 @@ def __init__(
 
         :param use_threads: If True, threads will be used when performing
             S3 transfers. If False, no threads will be used in
-            performing transfers; all logic will be run in the main thread.
+            performing transfers; all logic will be run in the current thread.
 
         :param max_bandwidth: The maximum bandwidth that will be consumed
             in uploading and downloading file content. The value is an integer