Fix border example
Fix border example

Fix following error.

cell.border_top
NoMethodError: undefined method `border_top' for #<RubyXL::Cell(0,0): nil, datatype=nil, style_index=2>

mezza/rubyXL,weshatheleopard/rubyXL,weshatheleopard/rubyXL,mezza/rubyXL,mezza/rubyXL,weshatheleopard/rubyXL