fix(bean_format): fix bean_format for beancount v3 (#297)

bean-format from beancount v3 requires '-' to read from stdin.
beancount v2 works both ways.

Author: stasjok

lua/null-ls/builtins/formatting/bean_format.lua

@@ -17,6 +17,7 @@ return h.make_builtin({
     filetypes = { "beancount" },
     generator_opts = {
         command = "bean-format",
+        args = { "-" },
         to_stdin = true,
     },
     factory = h.formatter_factory,