[C#] chore: Fix link to readme associated with C# package. (#1159)

## Linked issues

closes: #1155  (issue number)

## Details

Fix link to readme associated with C# package. 

## Attestation Checklist

- [x] My code follows the style guidelines of this project

- I have checked for/fixed spelling, linting, and other errors
- I have commented my code for clarity
- I have made corresponding changes to the documentation (we use
[TypeDoc](https://typedoc.org/) to document our code)
- My changes generate no new warnings
- I have added tests that validates my changes, and provides sufficient
test coverage. I have tested with:
  - Local testing
  - E2E testing in Teams
- New and existing unit tests pass locally with my changes

### Additional information

> Feel free to add other relevant information below

dotnet/packages/Microsoft.TeamsAI/Microsoft.TeamsAI/Microsoft.Teams.AI.csproj

@@ -50,7 +50,7 @@
   </ItemGroup>
 
   <ItemGroup>
-    <None Include="..\..\..\..\README.md" Pack="true" PackagePath="\" />
+    <None Include="..\README.md" Pack="true" PackagePath="\" />
   </ItemGroup>
 
 </Project>