DOC: Fix formatting of a code example

One of the code examples in the docstring of
`numpy.random.Generator.multivariate_normal()` was not being displayed
properly in the built documentation.

Author: eerovaher

numpy/random/_generator.pyx

@@ -3567,6 +3567,7 @@ cdef class Generator:
         (3, 3, 2)
 
         We can use a different method other than the default to factorize cov:
+
         >>> y = rng.multivariate_normal(mean, cov, (3, 3), method='cholesky')
         >>> y.shape
         (3, 3, 2)