Fixing a typo in an exception message. (#4643)

src/Microsoft.Health.Fhir.Api/Features/Filters/ValidateExportRequestFilterAttribute.cs

@@ -85,7 +85,7 @@ public override void OnActionExecuting(ActionExecutingContext context)
 
                 if (!requiredHeaderValueFound)
                 {
-                    throw new RequestNotValidException($"Missing required header value; header={PreferHeaderName},value={PreferHeaderValueOptional}.");
+                    throw new RequestNotValidException($"Missing required header value; header={PreferHeaderName},value={PreferHeaderValueRequired}.");
                 }
             }
             else