SYD: add formula and correct method's name

Change "sum-of-years' digits method" to "Sum-of-the-Years'-Digits method" that apparently is the correct name. Add formula as given at http://wiki.services.openoffice.org/w/index.php?title=Documentation/How_Tos/Calc:_SYD_function To calculate depreciation, SYD adds the year numbers during the asset's life; the year numbers taken backward and divided by that sum give the proportion of the total loss of value to assign to depreciation that year. SYD = (originalcost - salvagevalue) * (lifetime+1 - year) * 2 / ((lifetime+1) * lifetime)