Include buffer completion for rst files

This is useful when writing presentation slides.

after/ftplugin/rst.lua

@@ -1,10 +1,12 @@
-local opt = vim.opt
+local opt = vim.opt_local
 
 opt.wrap = true
 
 local cmp = require "cmp"
 local sources = cmp.get_config().sources
 
+-- TODO: confirm these aren't aleady in the list of sources to avoid duplicate suggestions.
+table.insert(sources, { name = "buffer" })
 table.insert(sources, { name = "path" })
 
 cmp.setup.buffer { sources = sources }