remove a period from a doxygen heading

The other headings don't have periods.

src/lib/crypt_ops/certs.md

 
-@page certificates Certificates in Tor.
+@page certificates Certificates in Tor
 
 We have, alas, several certificate types in Tor.
 
@@ -27,4 +27,3 @@ their associated keys.
 documents that include keys and which are signed by keys. You can
 consider these documents to be an additional kind of certificate if you
 want.)
-