Fix documentation - replace 'matrix' with 'vector' (pytorch#951)

Author: arnowaczynski

torch/_torch_docs.py

@@ -2544,7 +2544,7 @@
 Indices are ordered from left to right according to when each was sampled
 (first samples are placed in first column).
 
-If :attr:`input` is a vector, :attr:`out` is a matrix of size `num_samples`.
+If :attr:`input` is a vector, :attr:`out` is a vector of size `num_samples`.
 
 If :attr:`input` is a matrix with `m` rows, :attr:`out` is an matrix of shape `m \u00D7 n`.