[Bugfix] 2 keymaps assumed g:paredit_leader is identical to ','.

I guess that the `,` in the original code are used as the leader for paredit, but it can be different than `,`.
A simple solution, replacing `,S` by `g:paredit_leader.'S'`, is not sufficient because `g:paredit_leader.'S'` can behave as vim's initial `S` if we set `g:paredit_shortmaps` to a value other than 0. ~~Even if the value of `g:paredit_shortmaps` is 0, `g:paredit_leader.'S'` is mapped through `nnoremap` and therefore `S` in the original code behaves as vim's initial `S`.~~

This pull request proposes a fix for this problem by calling PareditSplice function directly.

### Postscript
The struck out sentence is wrong. The original code works as it is intended if `g:paredit_leader` is `,` and `g:paredit_shormaps` is `0`.