Fix documentation typo (#12519)

speficied -> specified

Author: chrisyeh96

sdk/storage/azure-storage-blob/azure/storage/blob/_download.py

@@ -258,7 +258,7 @@ class StorageStreamDownloader(object):  # pylint: disable=too-many-instance-attr
         The properties of the blob being downloaded. If only a range of the data is being
         downloaded, this will be reflected in the properties.
     :ivar int size:
-        The size of the total data in the stream. This will be the byte range if speficied,
+        The size of the total data in the stream. This will be the byte range if specified,
         otherwise the total size of the blob.
     """