fix: README is rst, not md

Author: texastony

README.rst

@@ -309,19 +309,19 @@ to your use-case in order to obtain peak performance.
 
 Thread safety
 ==========================
-The `EncryptionSDKClient` and all provided`CryptoMaterialsManager` are thread safe.
-But instances of `BaseKMSMasterKeyProvider` MUST not be shared between threads, 
-for the reasons outlined in (the boto3 docs)[https://boto3.amazonaws.com/v1/documentation/api/latest/guide/resources.html#multithreading-or-multiprocessing-with-resources].
+The ``EncryptionSDKClient`` and all provided ``CryptoMaterialsManager`` are thread safe.
+But instances of ``BaseKMSMasterKeyProvider`` MUST not be shared between threads, 
+for the reasons outlined in `the boto3 docs <https://boto3.amazonaws.com/v1/documentation/api/latest/guide/resources.html#multithreading-or-multiprocessing-with-resources>`_.
 
-Because the `BaseKMSMaterKeyProvider` creates a [new boto3 sessions](https://github.com/aws/aws-encryption-sdk-python/blob/08f305a9b7b5fc897d9cafac55fb98f3f2a6fe13/src/aws_encryption_sdk/key_providers/kms.py#L665-L674) per region,
+Because the ``BaseKMSMaterKeyProvider`` creates a `new boto3 sessions <https://github.com/aws/aws-encryption-sdk-python/blob/08f305a9b7b5fc897d9cafac55fb98f3f2a6fe13/src/aws_encryption_sdk/key_providers/kms.py#L665-L674>`_ per region,
 users do not need to create a client for every region in every thread;
-a new  `BaseKMSMasterKeyProvider` per thread is sufficient.
+a new  ``BaseKMSMasterKeyProvider`` per thread is sufficient.
 
-(The `BaseKMSMasterKeyProvider` is the internal parent class of all the KMS Providers.)
+(The ``BaseKMSMasterKeyProvider`` is the internal parent class of all the KMS Providers.)
 
-Finally, while the `CryptoMaterialsCache` is thread safe, 
+Finally, while the ``CryptoMaterialsCache`` is thread safe, 
 sharing entries in that cache across threads needs to be done carefully
-(see the !Note about partition name [in the API Docs](https://aws-encryption-sdk-python.readthedocs.io/en/latest/generated/aws_encryption_sdk.materials_managers.caching.html#aws_encryption_sdk.materials_managers.caching.CachingCryptoMaterialsManager)).
+(see the !Note about partition name `in the API Docs <https://aws-encryption-sdk-python.readthedocs.io/en/latest/generated/aws_encryption_sdk.materials_managers.caching.html#aws_encryption_sdk.materials_managers.caching.CachingCryptoMaterialsManager>`_).
 
 .. _AWS Encryption SDK: https://docs.aws.amazon.com/encryption-sdk/latest/developer-guide/introduction.html
 .. _cryptography: https://cryptography.io/en/latest/