Update instancenorm.py (#4171)

diff --git a/torch/nn/modules/instancenorm.py b/torch/nn/modules/instancenorm.py
index 3d5ed1d..9854727 100644
--- a/torch/nn/modules/instancenorm.py
+++ b/torch/nn/modules/instancenorm.py
@@ -37,7 +37,7 @@
 
 
 class InstanceNorm1d(_InstanceNorm):
-    r"""Applies Instance Normalization over a 2d or 3d input that is seen as a mini-batch.
+    r"""Applies Instance Normalization over a 3d input that is seen as a mini-batch.
 
     .. math::