Fix Git description to make base conf be N/A

I have corrected the incorrect description for Git.

-------
## Before

|  Application           | Base conf.     | Specific conf.      | Env. var.     |
| -----------------------|----------------|---------------------|---------------|
| configure_apt_proxy    | config.proxy.* | config.apt_proxy.*  | VAGRANT_APT_* |
| configure_git_proxy    | config.proxy.*  | config.git_proxy.*  | VAGRANT_GIT_* |
| configure_svn_proxy    | N/A            | config.svn_proxy.*  | VAGRANT_SVN_* |
| configure_yum_proxy    | config.proxy.* | config.yum_proxy.*  | VAGRANT_YUM_* |


:arrow_down: 
## After

|  Application           | Base conf.     | Specific conf.      | Env. var.     |
| -----------------------|----------------|---------------------|---------------|
| configure_apt_proxy    | config.proxy.* | config.apt_proxy.*  | VAGRANT_APT_* |
| configure_git_proxy    | N/A            | config.git_proxy.*  | VAGRANT_GIT_* |
| configure_svn_proxy    | N/A            | config.svn_proxy.*  | VAGRANT_SVN_* |
| configure_yum_proxy    | config.proxy.* | config.yum_proxy.*  | VAGRANT_YUM_* |

-------

[configure_git_proxy.rb#L18](https://github.com/tmatilai/vagrant-proxyconf/blob/master/lib/vagrant-proxyconf/action/configure_git_proxy.rb#L18)
```ruby
        # @return [Vagrant::Plugin::V2::Config] the configuration
        def config
          return @config if @config

          # Use only `config.git_proxy`, don't merge with the default config
          @config = @machine.config.git_proxy
          finalize_config(@config)
        end
```