CCM-14616: fix typo in the letter variant sent to Supplier API

Author: nhsd-angel-pastor

lambdas/print-sender-lambda/src/__tests__/app/print-sender.test.ts

@@ -74,7 +74,7 @@ describe('PrintSender', () => {
               url: mockPDFAnalysed.data.letterUri,
               clientId: mockPDFAnalysed.data.senderId,
               campaignId: 'digitalLetters',
-              letterVariantId: 'notify-digital-letter-standard',
+              letterVariantId: 'notify-digital-letters-standard',
             }),
           }),
         ]),

lambdas/print-sender-lambda/src/app/print-sender.ts

@@ -47,7 +47,7 @@ export class PrintSender {
           url: item.data.letterUri,
           clientId: item.data.senderId,
           campaignId: 'digitalLetters',
-          letterVariantId: 'notify-digital-letter-standard',
+          letterVariantId: 'notify-digital-letters-standard',
         },
       };