s/which/that/

Which is usually for descriptive clauses in technical writing, while
that is restrictive. Most of the time we are being being restrictive.
diff --git a/docs/hazmat/backends/interfaces.rst b/docs/hazmat/backends/interfaces.rst
index 5131ca1..cd64050 100644
--- a/docs/hazmat/backends/interfaces.rst
+++ b/docs/hazmat/backends/interfaces.rst
@@ -16,7 +16,7 @@
 
 .. class:: CipherBackend
 
-    A backend which provides methods for using ciphers for encryption
+    A backend that provides methods for using ciphers for encryption
     and decryption.
 
     The following backends implement this interface: