Fix GELU-related docstring formatting (#102845)
The docstring about GELU seems formatted incorrectly. The original docstring about GELU is rendered as below:
$$ \text{GELU}(x) = 0.5 * x * (1 + \text{Tanh}(\sqrt(2 / \pi) * (x + 0.044715 * x^3))) $$
where the square root of which part is confusing.
I double-checked the formula, which should be:
$$ \text{GELU}(x) = 0.5 * x * (1 + \text{Tanh}(\sqrt{2 / \pi} * (x + 0.044715 * x^3))) $$
where round brackets in resource code should be brace brackets.
> _formula in [original paper](https://arxiv.org/abs/1606.08415)_
> 
Pull Request resolved: https://github.com/pytorch/pytorch/pull/102845
Approved by: https://github.com/mikaylagawarecki
