doc: fix heading space bug in assert.md

Fixing a heading rendering bug

PR-URL: #35310
Reviewed-By: Richard Lau <[email protected]>
Reviewed-By: Rich Trott <[email protected]>

Author: tlhunter

doc/api/assert.md

@@ -160,7 +160,7 @@ added: v14.2.0
 
 This feature is currently experimental and behavior might still change.
 
-### `new assert.CallTracker()`
+### `new assert.CallTracker()`
 <!-- YAML
 added: v14.2.0
 -->