doc: ledger-mode: load lazily

If we use `(load "ledger-mode")`, we always load ledger-mode,
even if we are not editing a Ledger file; using `autoload`, we avoid
this and only load ledger-mode when we need it.
Merge pull request #328 from listx/master

doc: ledger-mode: load lazily

Thanks.  This should work fine.