README needs updating for new syntax

[caternuson]

This is no longer correct usage:

# Set a pixel in the origin 0,0 position.
matrix.pixel[0, 0] = 1
# Set a pixel in the middle 8, 4 position.
matrix.pixel[8, 4] = 1
# Set a pixel in the opposite 15, 7 position.
matrix.pixel[15, 7] = 1
matrix.show()

should be:

# Set a pixel in the origin 0,0 position.
matrix[0, 0] = 1
# Set a pixel in the middle 8, 4 position.
matrix[8, 4] = 1
# Set a pixel in the opposite 15, 7 position.
matrix[15, 7] = 1
matrix.show()

[caternuson]

Fixed with #24