dead link to code_of_conduct.md file in README.md
dead link to code_of_conduct.md file in README.md

link corrected in this pr.

found using this tool I created: https://githubreadmechecker.com/Home/Search?SingleRepoUri=https%3A%2F%2Fgithub.com%2Fgoogle%2Fclosure-compiler&User=&NumberOfReposToSearchFor=&MinStar=&UpdatedAfter=&SearchSort=2&SortAscDsc=1
nawawi/closure-compiler,ChadKillingsworth/closure-compiler,monetate/closure-compiler,monetate/closure-compiler,nawawi/closure-compiler,ChadKillingsworth/closure-compiler,monetate/closure-compiler,google/closure-compiler,ChadKillingsworth/closure-compiler,nawawi/closure-compiler,google/closure-compiler,google/closure-compiler,google/closure-compiler,nawawi/closure-compiler,monetate/closure-compiler,ChadKillingsworth/closure-compiler