Correct a couple of command errors.

doc/crypto/threads.pod

@@ -35,12 +35,12 @@ id_function(void) is a function that returns a thread ID. It is not
 needed on Windows nor on platforms where getpid() returns a different
 ID for each thread.
 
-=RETURN VALUES
+=head1 RETURN VALUES
 
 CRYPTO_num_locks() returns the required number of locks.
 The other functions return no values.
 
-=NOTE
+=head1 NOTE
 
 You can find out if OpenSSL was configured with thread support: