Fix syntax error in README.md

This is funny and caused me a bit of a headache the other day when I was checking out NERDCommenter for the first time.

I was incorrectly trying to toggle comments using just `<Leader>c` when I should have been using `<Leader>c<space>`.  Why was I doing that, you ask?  When github was parsing the repo's README.md file for display, it was incorrectly passing the `<space>` part through as an html tag so it wasn't showing up at all.  Thus, the docs on the redo homepage read `<Leader>c` for the `NERDComToggleComment` command.  Plain old `<Leader>c` was being interpreted in vim as the change lines command, so the lines I was selecting were being deleted.

At first, I was like "Man...this plugin doesn't work!"  Then, the next day, I gave it another try and noticed the error after reading the vim docs using `:help nerdcommenter`.