Update MANUAL-TESTING.md

updating manual test instructions

Author: colombod

MANUAL-TESTING.md

@@ -2,7 +2,7 @@ Manually testing the Polyglot Extension for VS Code
 ===================================================
 
 1. Install the latest VS Code - Insiders from [here](https://code.visualstudio.com/insiders/).
-2. Install the latest dotnet 7 SDK from [here](https://dotnet.microsoft.com/en-us/download).
+2. Install the latest dotnet 8 SDK from [here](https://dotnet.microsoft.com/en-us/download).
 3. From the internal signed build ([here](https://dev.azure.com/dnceng/internal/_build?definitionId=743&_a=summary)), find the latest passing build from `main`.
 4. Download the artifact `vscode/insiders/dotnet-interactive-vscode-*.vsix`.
 5. Launch VS Code - Insiders, open the extensions tab, then manually install the VSIX downloaded in the previous step.