Fix register usage comment in ARMv8 Montgomery sqr asm.

Fix register usage comment in ARMv8 Montgomery sqr asm.
In the bn_sqr8x_mont function, a comment describing the accumulation step for the partial product a[1]*a[0] incorrectly referenced the low part ('lo') of the product. The value being added to the temporary register t[2] is actually the high part ('hi') of that multiplication.
Correct the comment from "t[2]+lo(a[1]*a[0])" to "t[2]+hi(a[1]*a[0])".

Reviewed-by: Neil Horman <nhorman@openssl.org>
Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/29436)