[advice] Change conceal regular expression

In <https://hub.fastgit.org/lervag/vimtex/blob/master/autoload/vimtex/syntax/core.vim#L1376>

```vim
syntax match texMathSymbol "\\sum\>"               contained conceal cchar=∑
```

`"\\sum\>"` is too strict, it cannot match `\sum_{i = 1}^n` or `\sum^n_{i = 1}`

How about change it to `"\\sum\ze\(\>\|_\|\^\)"`?

Other operators are similar.

Thanks!