Fix link references in UPGRADING.md for latest remark

UPGRADING.md

@@ -1,16 +1,16 @@
 # Upgrade Guide
 
-This document describes breaking changes and how to upgrade. For a complete list of changes including minor and patch releases, please refer to the [`CHANGELOG`].
+This document describes breaking changes and how to upgrade. For a complete list of changes including minor and patch releases, please refer to the [`CHANGELOG`][changelog].
 
 ## v3
 
 Dropped support for node 4. No other breaking changes.
 
 ## v2
 
-This release drops Node.js 0.12, brings `memdown` up to par with latest [`levelup`] (v2) and [`abstract-leveldown`] (v4), simplifies serialization and removes global state.
+This release drops Node.js 0.12, brings `memdown` up to par with latest [`levelup`][levelup] (v2) and [`abstract-leveldown`][abstract-leveldown] (v4), simplifies serialization and removes global state.
 
-### Targets latest [`levelup`]
+### Targets latest [`levelup`][levelup]
 
 Usage has changed to:
 
@@ -29,7 +29,7 @@ const db = levelup('mydb', { db: memdown })
 
 ### No stringification of keys and values
 
-This means that in addition to Buffers, you can store any JS type without the need for [`encoding-down`]. This release also makes behavior consistent in Node.js and browsers. Please refer to the [README](./README.md) for a detailed explanation.
+This means that in addition to Buffers, you can store any JS type without the need for [`encoding-down`][encoding-down]. This release also makes behavior consistent in Node.js and browsers. Please refer to the [README](./README.md) for a detailed explanation.
 
 ### No global state or `location` argument
 
@@ -47,9 +47,9 @@ const db = window.mydb = levelup(memdown())
 
 ### No `null` batch operations
 
-Instead of skipping `null` operations, `db.batch([null])` will throw an error courtesy of [`abstract-leveldown`].
+Instead of skipping `null` operations, `db.batch([null])` will throw an error courtesy of [`abstract-leveldown`][abstract-leveldown].
 
-[`CHANGELOG`]: CHANGELOG.md
-[`abstract-leveldown`]: https://github.com/Level/abstract-leveldown
-[`levelup`]: https://github.com/Level/levelup
-[`encoding-down`]: https://github.com/Level/encoding-down
+[changelog]: CHANGELOG.md
+[abstract-leveldown]: https://github.com/Level/abstract-leveldown
+[levelup]: https://github.com/Level/levelup
+[encoding-down]: https://github.com/Level/encoding-down