Editing Fubini's theorem (section)

===Riemann integrals===
For [[Riemann integral]]s, Fubini's theorem is proven by refining the partitions along the x-axis and y-axis as to create a joint partition of the form <math>[x_i,x_{i+1}] \times [y_j,y_{j+1}]</math>, which is a partition over <math>X\times Y</math>. This is used to show that the double integrals of either order are equal to the integral over <math>X\times Y</math>.