Don't use whirlpool as an example (#4053)

Both because it's weirdo crypto, but also because we don't even support it.

Adhere to our documented policy of using good crypto for all examples
diff --git a/docs/hazmat/primitives/cryptographic-hashes.rst b/docs/hazmat/primitives/cryptographic-hashes.rst
index 521ae09..7e23ce9 100644
--- a/docs/hazmat/primitives/cryptographic-hashes.rst
+++ b/docs/hazmat/primitives/cryptographic-hashes.rst
@@ -175,7 +175,7 @@
         :type: str
 
         The standard name for the hash algorithm, for example: ``"sha256"`` or
-        ``"whirlpool"``.
+        ``"blake2b"``.
 
     .. attribute:: digest_size