remove extra blank line in README
remove extra blank line in README

pombredanne/spdx-expression-validate.js,kasicka/spdx.js,kemitchell/spdx-expression-validate.js,kemitchell/spdx.js,pombredanne/spdx.js,jslicense/spdx-expression-parse.js,pombredanne/spdx-expression-parse.js,kemitchell/spdx-expression-parse.js