Generalize requirement to just "an API token"

We now have multiple ways to get an API token so its nicer to keep this
statement general.

Author: jparise

README.md

@@ -7,7 +7,7 @@ builds upon the foundational git functionality provided by [Fugitive][].
 * Syntax highlighting for Arcanist templates and configuration files
 * `:Gbrowse` support for browsing Phabricator URLs from the current buffer
 * Completion support for users and projects in Arcanist diff templates using
-  [`<C-X><C-O>`][compl-omni] (requires [curl][] and `g:phabricator_api_token`)
+  [`<C-X><C-O>`][compl-omni] (requires [curl][] and an API token)
 
 [fugitive]: https://github.com/tpope/vim-fugitive
 [compl-omni]: http://vimdoc.sourceforge.net/htmldoc/insert.html#compl-omni