Update plugin runtime filename

I am still wrestling with getting this installed, but I believe this is an error in the documentation.  There is no file ftdetect/apexcode.vim.  The file 'apexcode.vim' lives in the 'ftplugin' directory.  Changing these two lines of code allowed the plugin to load its scripts and run (although I'm still unable to successfully pull down my project files).
one more ftdetect/ file name change fix