Fixes bug in formula for wkhtmltopdf.

Formula for wkhtmltopdf was passing inreplace a Symbol where it expected a String.

    Error: can't convert Symbol into String
    /usr/local/Library/Homebrew/utils/inreplace.rb:12:in 'gsub!'
    /usr/local/Library/Homebrew/utils/inreplace.rb:12:in 'inreplace'
    /usr/local/Library/Homebrew/utils/inreplace.rb:4:in 'each'
    /usr/local/Library/Homebrew/utils/inreplace.rb:4:in 'inreplace'
    /usr/local/Library/Formula/wkhtmltopdf.rb:23:in 'install'
    /usr/local/Library/Homebrew/build.rb:170:in 'install'
    /usr/local/Library/Homebrew/formula.rb:239:in 'brew'
    /usr/local/Library/Homebrew/formula.rb:600:in 'stage'
    /usr/local/Library/Homebrew/extend/fileutils.rb:21:in 'mktemp'
    /usr/local/Library/Homebrew/formula.rb:596:in 'stage'
    /usr/local/Library/Homebrew/formula.rb:234:in 'brew'
    /usr/local/Library/Homebrew/build.rb:149:in 'install'
    /usr/local/Library/Homebrew/build.rb:43:in 'main'
    /usr/local/Library/Homebrew/build.rb:12
    /usr/local/Library/Formula/wkhtmltopdf.rb:11