Address Issue aws#331 Typo in docstring for StreamDecryptor

Address Issue aws#331 Typo in docstring for StreamDecryptor by correctly referring to the class as a decryption utility as compared to encryption utility. (Utility is probably not the right word...)

Author: texastony

src/aws_encryption_sdk/streaming_client.py

@@ -734,7 +734,7 @@ class DecryptorConfig(_ClientConfig):
 
 
 class StreamDecryptor(_EncryptionStream):  # pylint: disable=too-many-instance-attributes
-    """Provides a streaming encryptor for encrypting a stream source.
+    """Provides a streaming decryptor for decrypting a stream source.
     Behaves as a standard file-like object.
 
     .. note::