Update multivariate.py

Updated the docstrings for better clarity. Replaced "log-likelihood" with "distribution" to accurately describe the functionality, as the class provides more than just log-likelihood, including cdf, mean, and random methods.

Author: tanishy7777

pymc/distributions/multivariate.py

@@ -194,7 +194,7 @@ def quaddist_chol(value, mu, cov):
 
 class MvNormal(Continuous):
     r"""
-    Multivariate normal log-likelihood.
+    Multivariate normal distribution.
 
     .. math::