gnu: python-pbkdf2: Fix typo in description.

* gnu/packages/python-xyz.scm (python-pbkdf2)[description]: Fix typo.

gnu/packages/python-xyz.scm

@@ -14443,7 +14443,7 @@ (define-public python-pbkdf2
 is part of the RSA Public Key Cryptography Standards series.  The provided
 implementation takes a password or a passphrase and a salt value (and
 optionally a iteration count, a digest module, and a MAC module) and provides
-a file-like object from which an arbitrarly-sized key can be read.")
+a file-like object from which an arbitrarily-sized key can be read.")
     (license license:expat)))
 (define-public python2-pbkdf2