Resolve over command syntax error which causes 'make install' to fail

Reviewed-by: NMatt Caswell <matt@openssl.org>
Reviewed-by: NRich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/1312)

doc/crypto/BIO_set_callback.pod

@@ -40,15 +40,24 @@ BIO_callback_fn() is the type of the callback function. The meaning of each
 argument is described below:
 
 =over
+
+=item B<b>
+
 The BIO the callback is attached to is passed in B<b>.
 
+=item B<oper>
+
 B<oper> is set to the operation being performed. For some operations
 the callback is called twice, once before and once after the actual
 operation, the latter case has B<oper> or'ed with BIO_CB_RETURN.
 
+=item B<argp> B<argi> B<argl>
+
 The meaning of the arguments B<argp>, B<argi> and B<argl> depends on
 the value of B<oper>, that is the operation being performed.
 
+=item B<ret>
+
 B<ret> is the return value that would be returned to the
 application if no callback were present. The actual value returned
 is the return value of the callback itself. In the case of callbacks