Fix typo in version number

Author: Apanatshka

content/release/note/2.5.21.md

@@ -1,6 +1,6 @@
-# Spoofax 2.5.20 (23-07-2023)
+# Spoofax 2.5.21 (23-07-2023)
 
-Spoofax 2.5.20 updates multiple dependencies, fixes a few bugs, and introduces a new required field in the `metaborg.yaml` file.
+Spoofax 2.5.21 updates multiple dependencies, fixes a few bugs, and introduces a new required field in the `metaborg.yaml` file.
 
 See the corresponding [migration guide](../migrate/2.5.21.md) for migrating from Spoofax 2.5.20 (or below) to Spoofax 2.5.21.