Update CHANGELOG.md

Author: nevrome

CHANGELOG.md

@@ -2,4 +2,5 @@
 - V 2.0.1: Added a field definition for the POSEIDON.yml file, made UTF-8 encoding mandatory, more detailed definition of .janno columns
 - V 2.1.0: Added checksums and the paths to the README and CHANGELOG files in the POSEIDON.yml, cleaned the schema definition and removed DAG internal procedures that do not belong here
 - V 2.2.0: A lot more freedom for the .janno file while remaining fully backwards compatible
-- V 2.3.0: Introduced janno-columns for source accession numbers, and a package-level entry called snpset.
+- V 2.3.0: Introduced janno-columns for source accession numbers, and a package-level entry called snpset.
+- V 2.3.1: 1240K instead of 1240k everywhere