Fixed a readme extension (#16049)

Author: psfinaki

tests/walkthroughs/sdk-script-manual-tests/README.md.txt renamed to tests/walkthroughs/sdk-script-manual-tests/README.md

@@ -82,4 +82,4 @@ dotnet artifacts\bin\fsi\Debug\netcoreapp3.1\fsi.exe c:\misc\tests\neutral-scrip
 dotnet artifacts\bin\fsi\Debug\netcoreapp3.1\fsi.exe c:\misc\tests\neutral-script-reference-netfx-script.fsx
 dotnet artifacts\bin\fsi\Debug\netcoreapp3.1\fsi.exe c:\misc\tests\neutral-script-reference-neutral-script.fsx
 
-```
+```