Fix issue in changelog markdown format

Author: hanefi

CHANGELOG.md

@@ -2,8 +2,8 @@
 ==========
 v2.15
 - Updated for PostgreSQL 13
-- [PR #73]((https://github.com/citusdata/postgresql-hll/pull/73), Support parallel queries / aggregates
-- [PR #107]((https://github.com/citusdata/postgresql-hll/pull/107), Fixed out of range bug
+- [PR #73](https://github.com/citusdata/postgresql-hll/pull/73), Support parallel queries / aggregates
+- [PR #107](https://github.com/citusdata/postgresql-hll/pull/107), Fixed out of range bug
 
 2019-11-15
 ==========