fix(rosetta): capitalise Quickstart for consistency (#175)

## Summary

- Fixes "Quick start" → "Quickstart" in `rosetta.md` to match the
single-word form used across the rest of the docs

## Sync recommendation

hand-written

Author: marc0olo

docs/guides/digital-assets/rosetta.md

@@ -129,7 +129,7 @@ Pull the official image:
 docker pull dfinity/ic-icrc-rosetta-api
 ```
 
-**Quick start**: connects to the TICRC1 test token (`3jkp5-oyaaa-aaaaj-azwqa-cai`):
+**Quickstart**: connects to the TICRC1 test token (`3jkp5-oyaaa-aaaaj-azwqa-cai`):
 
 ```bash
 docker run \