docs: refine sendMany endpoint description

Clarify the SendManyRoute description to better reflect that the
endpoint sends a transaction to one or more recipients.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: starfy84

src/masterBitgoExpress/routers/sendManyRoute.ts

@@ -234,5 +234,5 @@ export const SendManyRoute = httpRoute({
     body: SendManyRequest,
   }),
   response: SendManyResponse,
-  description: 'Send many transactions',
+  description: 'Send a transaction to one or more recipients',
 });