[skip ci] chore: minor update to release notes

docs/releases/v2.3.0.md

@@ -19,6 +19,6 @@
 - fixed a few typos
 - fixed the reference space name of `ICBM 152 2009c Nonlinear Asymmetric`
 
-# Under the hood stuff
+## Under the hood stuff
 
 - Updated how dataset retrieval work. It will now query on a region basis