Fixed "stong" to "strong" typo

- fixes SharePoint#4407

Author: andrewconnell

docs/declarative-customization/site-design-json-schema.md

@@ -757,7 +757,7 @@ Use the **setSiteBranding** verb to specify the navigation layout, the header la
 
 - **navigationLayout** – Specify the navigation layout as Cascade or Megamenu
 - **headerLayout** – Specify the header layout as Standard or Compact
-- **headerBackground** – Specify the header background as None, Neutral, Soft or Stong
+- **headerBackground** – Specify the header background as None, Neutral, Soft or Strong
 
 #### Example