doc: update catalina xcode cli tools download link (#2044)
doc: update catalina xcode cli tools download link (#2044)

PR-URL: https://github.com/nodejs/node-gyp/pull/2044
Reviewed-By: Rod Vagg <f9f03903429036ef609ed7ca822fddfa76155c29@vagg.org>
Reviewed-By: Jiawen Geng <8f5a7089e1809fdab8e8d97ec73a77a8b0770d74@gmail.com>
nodegit/node-gyp,nodejs/node-gyp,nodejs/node-gyp,nodegit/node-gyp,nodegit/node-gyp,nodejs/node-gyp,nodejs/node-gyp,nodegit/node-gyp,nodejs/node-gyp,nodegit/node-gyp