Add pr link in changelog

Signed-off-by: Alibi Shalgymbay <aliby.bbb@gmail.com>

CHANGELOG.md

@@ -17,7 +17,7 @@ We use *breaking :warning:* to mark changes that are not backward compatible (re
 
 ### Added
 
-- [#](https://github.com/thanos-io/thanos/pull/) Store Gateway: Added `--store.index-header-lazy-reader-idle-delete-timeout` to periodically delete idle index header files, when lazy reader is enabled and idle timeout for unloading is set > 0.
+- [#7118](https://github.com/thanos-io/thanos/pull/7118) Store Gateway: Added `--store.index-header-lazy-reader-idle-delete-timeout` to periodically delete idle index header files, when lazy reader is enabled and idle timeout for unloading is set > 0.
 
 ### Changed