Clarify input dimensions for pcolormesh in documentation

[jkunimune]

The plotting data tutorial states that the x, y, and data inputs to pcolormesh should be of the same size, but that's not correct. As per the Basemap.pcolormesh docstring, x and y should have dimensions one greater than those of data; the code will still run if they have the same size but the last row and column get dropped.