Fixed minor error in readme
Fixed minor error in readme

'git clean -fdx' only erases untracked files. It doesn't erase
all changes (as incorrectly stated in one place).

kyroskoh/lets_code_javascript,halcwb/lets_code_javascript,halcwb/lets_code_javascript,kyroskoh/lets_code_javascript,halcwb/lets_code_javascript,kyroskoh/lets_code_javascript