Update changelog

Author: pinin4fjords

CHANGELOG.md

@@ -21,6 +21,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - [PR #1521](https://github.com/nf-core/rnaseq/pull/1521) - Updated Perl conda package version for local module gtf2bed for Arm compatibility.
 - [PR #1550](https://github.com/nf-core/rnaseq/pull/1550) - Simplify `SummarizedExperiment` outputs.
 - [PR #1553](https://github.com/nf-core/rnaseq/pull/1553) - Make jobs automatically resubmit for exit code 175
+- [PR #1556](https://github.com/nf-core/rnaseq/pull/1556) - Update index docs to remove references to 'indexing only mode'
 
 # 3.18.0 - 2024-12-19