fix symmetric encryption interfaces heading (#3676)

diff --git a/docs/hazmat/primitives/symmetric-encryption.rst b/docs/hazmat/primitives/symmetric-encryption.rst
index 8c6087e..7e05acd 100644
--- a/docs/hazmat/primitives/symmetric-encryption.rst
+++ b/docs/hazmat/primitives/symmetric-encryption.rst
@@ -435,7 +435,7 @@
     **Padding is required when using this mode.**
 
 Interfaces
-----------
+~~~~~~~~~~
 
 .. currentmodule:: cryptography.hazmat.primitives.ciphers