fix: capitalize API Reference per Vale style guide

Co-Authored-By: Colton Berry <[email protected]>

Author: devin-ai-integration[bot]

fern/products/docs/pages/navigation/tab-variants.mdx

@@ -142,7 +142,7 @@ Tab variants are particularly useful for:
 
 ## Variants vs. tabs
 
-Use **variants** when you want to show different perspectives or approaches to the same content area. Use **tabs** when you want to organize completely different sections of your documentation (like separating guides from API reference).
+Use **variants** when you want to show different perspectives or approaches to the same content area. Use **tabs** when you want to organize completely different sections of your documentation (like separating guides from API Reference).
 
 ```yaml title="When to use variants"
 # Good: Different approaches to the same topic