Update README.md

This pull request addresses the issue of a broken link in the notebook execution by replacing it with a functional one.

README.md

@@ -1,5 +1,5 @@
 [![Notebook Execution and Validation](https://github.com/spacetelescope/hst_notebooks/actions/workflows/ci_runner.yml/badge.svg)](https://github.com/spacetelescope/hst_notebooks/actions/workflows/ci_runner.yml)
-[![Scheduled Notebook Execution](https://github.com/spacetelescope/hst_notebooks/actions/workflows/ci_scheduled.yml/badge.svg)](https://github.com/spacetelescope/hst_notebooks/actions/workflows/ci_scheduled.yml)
+[![Scheduled Notebook Execution](https://github.com/spacetelescope/hst_notebooks/actions/workflows/ci_nightly.yml/badge.svg)](https://github.com/spacetelescope/hst_notebooks/actions/workflows/ci_nightly.yml)
 
 
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