[DOC] fix loc/scale parameter names in TDistribution docstring (#61)

Fixes location-scale parameter names in the docstring of
`TDistribution`, the parameter names in the docstring were inconsistent
with the actual parameter names.

FYI @Alex-JG3, before we release we can easily switch that either way -
do you have a preference?
My default action is always to fix the docstring if the logic and the
docstring disagree.

skpro/distributions/t.py

@@ -15,9 +15,9 @@ class TDistribution(BaseDistribution):
 
     Parameters
     ----------
-    mean : float or array of float (1D or 2D)
+    mu : float or array of float (1D or 2D)
         mean of the t-distribution distribution
-    sd : float or array of float (1D or 2D), must be positive
+    sigma : float or array of float (1D or 2D), must be positive
         standard deviation of the t-distribution distribution
     df : float or array of float (1D or 2D), must be positive
         Degrees of freedom of the t-distribution