Fix wrong snippet commands
Fix wrong snippet commands

I modify snippets for Integral and Partial derivative for some typo.
E.g., lack of \ and the argument number.

honza/vim-snippets,adam-e-trepanier/vim-snippets,phatograph/vim-snippets,kevinkirkup/vim-snippets,craigp/vim-snippets,craigp/vim-snippets,adam-e-trepanier/vim-snippets,honza/vim-snippets,kevinkirkup/vim-snippets,phatograph/vim-snippets