Update openapi.yaml

Co-authored-by: Matthias Mohr <matthias@mohr.ws>

Author: soxofaan

openapi.yaml

@@ -2262,7 +2262,7 @@ paths:
               schema:
                 $ref: '#/components/schemas/money'
             OpenEO-Identifier:
-              description: Identifier associated with the synchronous processing request.
+              description: Optionally, an identifier associated with the synchronous processing request.
               schema:
                 type: string
                 pattern: '^[\w\-\.~]+$'