Arm backend: Fix typo in tutorial for Arm Corstone-320 (pytorch#15255)

### Summary
Fixing typo in [Arm Ethos-U NPU Backend
Tutorial](https://docs.pytorch.org/executorch/main/tutorial-arm-ethos-u.html).

### Test plan
No test needed.

Author: NostalgiaJohn

docs/source/tutorial-arm-ethos-u.md

@@ -31,7 +31,7 @@ This tutorial serves as an introduction to using ExecuTorch to deploy PyTorch mo
 
 To successfully complete this tutorial, you will need a Linux machine with aarch64 or x86_64 processor architecture, or a macOS™ machine with Apple® Silicon.
 
-To enable development without a specific development board, we will be using a [Fixed Virtual Platform (FVP)](https://www.arm.com/products/development-tools/simulation/fixed-virtual-platforms), simulating [Arm® Corstone™-300](https://developer.arm.com/Processors/Corstone-300)(cs300) and [Arm® Corstone™-300](https://developer.arm.com/Processors/Corstone-320)(cs320)systems. Think of it as virtual hardware.
+To enable development without a specific development board, we will be using a [Fixed Virtual Platform (FVP)](https://www.arm.com/products/development-tools/simulation/fixed-virtual-platforms), simulating [Arm® Corstone™-300](https://developer.arm.com/Processors/Corstone-300)(cs300) and [Arm® Corstone™-320](https://developer.arm.com/Processors/Corstone-320)(cs320)systems. Think of it as virtual hardware.
 
 ### Software