fix typo in dotnet-install file

This was needed to rebuild the CLI with the updated Roslyn NuGet packages.

Thanks goes to @akoken for the fix.

Author: eerhardt

scripts/obtain/dotnet-install.sh

@@ -668,4 +668,4 @@ else
     say "Binaries of dotnet can be found in $bin_path"
 fi
 
-say "Installation finished successfuly."
+say "Installation finished successfully."