Update stepper.md (#1524)

Fix the rendered example under `## Basic stepper` to remove a duplication of the step title

Author: vishaangelova

docs/syntax/stepper.md

@@ -20,7 +20,6 @@ npm install
 ::::
 
 ::::{step} Build
-### Build
 Then build the project.
 ```shell
 npm run build
@@ -225,4 +224,4 @@ Content here...
 ```
 ## Dynamic heading levels
 
-Stepper step titles automatically adjust their heading level based on the preceding heading in the document, ensuring proper document hierarchy and semantic structure.
+Stepper step titles automatically adjust their heading level based on the preceding heading in the document, ensuring proper document hierarchy and semantic structure.