fix typo in comment (#4019)

diff --git a/src/cryptography/utils.py b/src/cryptography/utils.py
index baa7b7a..d69ed89 100644
--- a/src/cryptography/utils.py
+++ b/src/cryptography/utils.py
@@ -12,7 +12,7 @@
 
 
 # We use a UserWarning subclass, instead of DeprecationWarning, because CPython
-# decided deprecation warnings should be invisble be default.
+# decided deprecation warnings should be invisble by default.
 class CryptographyDeprecationWarning(UserWarning):
     pass