DOC: fix simplex math for scipy.stats.dirichlet documentation

[AtsushiSakai]

Reference issue

fix #12415

What does this implement/fix?

fixed simplex math for scipy.stats.dirichlet documentation based on https://en.wikipedia.org/wiki/Dirichlet_distribution.
And added a comment about error raising.

[AtsushiSakai]

I confirmed the current implementation satisfies the new simplex math.

>>> from scipy.stats import dirichlet
>>> import numpy as np
>>> quantiles = np.array([0.2, 0.2, 0.6])
>>> alpha = np.array([0.4, 5, 15])  
>>> dirichlet.pdf(quantiles, alpha)
0.2843831684937255
>>> quantiles = np.array([0.2, 0.2, 0.5])
>>> dirichlet.pdf(quantiles, alpha)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Library/Frameworks/Python.framework/Versions/3.8/lib/python3.8/site-packages/scipy/stats/_multivariate.py", line 1483, in pdf
    x = _dirichlet_check_input(alpha, x)
  File "/Library/Frameworks/Python.framework/Versions/3.8/lib/python3.8/site-packages/scipy/stats/_multivariate.py", line 1286, in _dirichlet_check_input
    raise ValueError("The input vector 'x' must lie within the normal "
ValueError: The input vector 'x' must lie within the normal simplex. but np.sum(x, 0) = 0.9.
>>> quantiles = np.array([0.6, -0.2, 0.6])
>>> dirichlet.pdf(quantiles, alpha)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Library/Frameworks/Python.framework/Versions/3.8/lib/python3.8/site-packages/scipy/stats/_multivariate.py", line 1483, in pdf
    x = _dirichlet_check_input(alpha, x)
  File "/Library/Frameworks/Python.framework/Versions/3.8/lib/python3.8/site-packages/scipy/stats/_multivariate.py", line 1268, in _dirichlet_check_input
    raise ValueError("Each entry in 'x' must be greater than or equal "
ValueError: Each entry in 'x' must be greater than or equal to zero.

[WarrenWeckesser]

I suggested a small change to fix how the LaTeX is rendered.

[WarrenWeckesser]

👍

[WarrenWeckesser]

Thanks @AtsushiSakai, merged.