Fix documentation for g:tmru#world.cache_var

[blueyed]

The README mentions the following to restore filters across calls to :TRecentlyUsedFiles:

let g:tmru_world.cache_var = 'g:tmru_cache'
let g:tmru_world.restore_from_cache = ['filter']

This does not work for two reasons: it should be g:tmru#world and g:tmru#world needs to be accessed/read once, before it is available.

Please fix the documentation accordingly.

I am using the following now in ~/.vim/after/plugin/tmru.vim, but would like a solution where I could have it in ~/.vimrc directly:

let foo = g:tmru#world
let g:tmru#world.cache_var = 'g:tmru_cache' " used for g:tmru#world.restore_from_cache
unlet foo