Update `--nodedir` description in README.
Update `--nodedir` description in README.

The description erroneously stated that it should point the node binary.
It needs to point to the node source code.

PR-URL: https://github.com/nodejs/node-gyp/pull/1372
Reviewed-By: Joyee Cheung <c24c8f580289b8360adc3fe49d66727180fe8aa3@gmail.com>
Reviewed-By: Richard Lau <8193d74c06330af7778ef55510160e4e561ed6a1@uk.ibm.com>

TooTallNate/node-gyp,TooTallNate/node-gyp,refack/node-gyp,refack/node-gyp,TooTallNate/node-gyp,TooTallNate/node-gyp,refack/node-gyp,refack/node-gyp,TooTallNate/node-gyp,refack/node-gyp