Change default side of filled zone to F&B to match comment

[HWXLR8]

Comment for filled zone says:

// Params:
//    side: default is 'F&B' for both Front and Back

but the code sets the side to front:

  params: {
    side: 'F',

unfortunately we need to escape the ampersand or else the final output to kicad will result in NaN.Cu after evaluating F&B.

Also fixes a typo in p.priority which stops the parameter from being applied.