doc: mappings use <LocalLeader> prefix, not <Leader>

If you have changed the mapleader/<Leader> key in vim, the plugin
mappings do not work. This is because the plugin actually uses
<LocalLeader>, not <Leader> like the documentation says. Fix
documentation.

Also, list the mapping for :GraphvizCompilerToLaTeX in the MAPPINGS
section too. Although the mapping is listed in the DOT2TEX section, I
think it is good to have all mappings exported by the plugin listed in
the MAPPINGS section.
doc: fix tex2dot => dot2tex typos