Update CHANGELOG.md with R1C1 conversion change

Author: ndench

CHANGELOG.md

@@ -29,6 +29,7 @@ and this project adheres to [Semantic Versioning](https://semver.org).
 - Nothing.
 
 ### Fixed
+- Correctly handle absolute A1 references when converting to R1C1 format [PR #2060](https://github.com/PHPOffice/PhpSpreadsheet/pull/2060)
 - Correct default fill style for conditional without a pattern defined [Issue #2035](https://github.com/PHPOffice/PhpSpreadsheet/issues/2035) [PR #2050](https://github.com/PHPOffice/PhpSpreadsheet/pull/2050)
 - Fixed issue where array key check for existince before accessing arrays in Xlsx.php. [PR #1970](https://github.com/PHPOffice/PhpSpreadsheet/pull/1970)
 - Fixed issue with quoted strings in number format mask rendered with toFormattedString() [Issue 1972#](https://github.com/PHPOffice/PhpSpreadsheet/issues/1972) [PR #1978](https://github.com/PHPOffice/PhpSpreadsheet/pull/1978)