docs: fix typo (#458)

docs/secret.rst

@@ -7,7 +7,7 @@ Secret key encryption (also called symmetric key encryption) is analogous to a
 safe. You can store something secret through it and anyone who has the key can
 open it and view the contents. :class:`~nacl.secret.SecretBox` functions as
 just such a safe, and like any good safe any attempts to tamper with the
-contents is easily detected.
+contents are easily detected.
 
 Secret key encryption allows you to store or transmit data over insecure
 channels without leaking the contents of that message, nor anything about it