fdo#71128 arguments of calc function MMULT have same name

Arguments 1 and 2 of MMULT have the same name in the function
wizard, which is both confusing for the user and inconstent
with other argument names for matrix functions.
Patch simply changes the names to match the description of
the arguments of MMULT.

Change-Id: I9fc35907133aa61248182bd3b253a7f90ba7cd2c
Reviewed-on: https://gerrit.libreoffice.org/6526
Reviewed-by: Eike Rathke <erack@redhat.com>
Tested-by: Eike Rathke <erack@redhat.com>