[DOC] Fix latex in trustworthiness docstring (scikit-learn#8832)

Curly parens were misplaced resulting in an empty sum when rendered.

Author: lmcinnes

sklearn/manifold/t_sne.py

@@ -441,7 +441,7 @@ def trustworthiness(X, X_embedded, n_neighbors=5, precomputed=False):
     .. math::
 
         T(k) = 1 - \frac{2}{nk (2n - 3k - 1)} \sum^n_{i=1}
-            \sum_{j \in U^{(k)}_i (r(i, j) - k)}
+            \sum_{j \in U^{(k)}_i} (r(i, j) - k)
 
     where :math:`r(i, j)` is the rank of the embedded datapoint j
     according to the pairwise distances between the embedded datapoints,