docs: fix dangling sentence fragment in PrivateUse1 (#3898)

Fixes #3849 

## Description
In the file PrivateUse1, there existed a dangling sentence fragment that
appeared to be forgotten:

> you are an advanced user of PyTorch.

This PR removes that.

Author: abhiman123

advanced_source/privateuseone.rst

@@ -4,7 +4,6 @@ Facilitating New Backend Integration by PrivateUse1
 In this tutorial we will walk through some necessary steps to integrate a new backend
 living outside ``pytorch/pytorch`` repo by ``PrivateUse1``. Note that this tutorial assumes that
 you already have a basic understanding of PyTorch.
-you are an advanced user of PyTorch.
 
 .. note::