Fix symbols in options Hash examples (:repository, :index)

Fix Readme.md to use correct symbols for options Hash.

According to `base.rb` in v.1.2.6, the options Hash symbols are `:repository` instead of `:git_dir`, and `:index` instead of `:index_file`, as in:
https://github.com/schacon/ruby-git/blob/v1.2.6/lib/git/base.rb#L25
https://github.com/schacon/ruby-git/blob/v1.2.6/lib/git/base.rb#L72