Fix doc. about linear from/to Cartesian indices

[emmt]

Since Julia (ordinary) arrays have 1-based indices and are in column-major order, the linear index k and the Cartesian index (i,j) of an element in a m×n Julia array are related by: k = i + m*(j - 1). The examples of conversion between linear and Cartesian indices in the doc. use incorrect formulae although the results were correct in the specific element at (1,3). This can be easily checked for other indices than (1,3) with a simple 2×3 array (as in the examples) built by A = reshape(collect(1:6),2,3).