Change in GitHub link for the curl line in installation instructions

[alexbhandari]

The link to raw.github.com redirects to raw.githubusercontent.com, which causes curl to return nothing.

You might want to update the readme to

curl https://raw.githubusercontent.com/davidjrice/prezto_powerline/master/prompt_powerline_setup > ~/.zsh.prompts/prompt_powerline_setup

[erikreyna]

+1: this just fixed the problem I was having with zsh: bad math expression: operand expected at %F{135}%n%...'` showing up after restarting the shell.