Remove preview URL reference from runtime-overview.md (#931)

Summary:
Pull Request resolved: #931

This was the only reference I found to the preview hostname in the tree.

Reviewed By: mergennachin

Differential Revision: D50307683

fbshipit-source-id: b20847d8b5ca6a13f881708fa8bad809b29dddb7

Author: dbort

docs/source/runtime-overview.md

@@ -11,8 +11,8 @@ Works](intro-how-it-works.md).
 At the highest level, the ExecuTorch runtime is responsible for:
 
 * Loading binary `.pte` program files that were generated by the
-  [`to_executorch()`](https://deploy-preview-646--resplendent-gnome-14e531.netlify.app/tutorials/export-to-executorch-tutorial)
-  step of the model-lowering process.
+  [`to_executorch()`](./tutorials/export-to-executorch-tutorial) step of the
+  model-lowering process.
 * Executing the series of instructions that implement a lowered model.
 
 Note that as of late 2023, the ExecuTorch runtime only supports model inference,