Update global.json to use latest .NET Core 3 SDK available

Ix.NET/Source/global.json

@@ -1,6 +1,6 @@
 {
   "sdk": {
-    "version": "3.0.100-preview-010184"
+    "version": "3.0.100-preview"
   },
   "msbuild-sdks": {
     "MSBuild.Sdk.Extras": "2.0.0-preview.14"