[FIX] Tex conflicting snippets for `it`
[FIX] Tex conflicting snippets for `it`

The `it` snippet was already used for `/item`. PR #1243 reused it for italics, leading to a conflict.
This PR uses `ita` for italics, as it is already the case in  [/snippets/tex.snippets](https://github.com/honza/vim-snippets/blob/master/snippets/tex.snippets).
kevinkirkup/vim-snippets,vito-c/vim-snippets,vito-c/vim-snippets,kevinkirkup/vim-snippets,honza/vim-snippets,adam-e-trepanier/vim-snippets,honza/vim-snippets,needle-wang/vim-snippets,phatograph/vim-snippets,craigp/vim-snippets,phatograph/vim-snippets,needle-wang/vim-snippets,craigp/vim-snippets,adam-e-trepanier/vim-snippets