fix: logit sigmoid function typo

from `(1 / (1 + exp(x))` to `(1 / (1 + exp(-x))`

Author: guin0x

pytensor/tensor/math.py

@@ -1416,7 +1416,7 @@ def iv(v, x):
 
 @scalar_elemwise
 def sigmoid(x):
-    """Logistic sigmoid function (1 / (1 + exp(x)), also known as expit or inverse logit"""
+    """Logistic sigmoid function (1 / (1 + exp(-x)), also known as expit or inverse logit"""
 
 
 expit = sigmoid