Typo fixed

Reviewed-by: NMatt Caswell <matt@openssl.org>
Reviewed-by: NRich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2075)

doc/man3/BUF_MEM_new.pod

@@ -44,7 +44,7 @@ size.
 BUF_MEM_grow_clean() is similar to BUF_MEM_grow() but it sets any free'd
 or additionally-allocated memory to zero.
 
-BUF_reverse() reverses B<size> bytes at B<in> into B<out>.  If B<out>
+BUF_reverse() reverses B<size> bytes at B<in> into B<out>.  If B<in>
 is NULL, the array is reversed in-place.
 
 =head1 RETURN VALUES