Port a few cr.yp.to links to be HTTPS

Because researching crypto should probably be secure.
diff --git a/docs/faq.rst b/docs/faq.rst
index 0b7bdce..10c8656 100644
--- a/docs/faq.rst
+++ b/docs/faq.rst
@@ -14,5 +14,5 @@
 
 If you prefer NaCl's design, we highly recommend `PyNaCl`_.
 
-.. _`NaCl`: http://nacl.cr.yp.to/
+.. _`NaCl`: https://nacl.cr.yp.to/
 .. _`PyNaCl`: https://pynacl.readthedocs.org