Fix duplicate "(default: 3)" in --max-concurrent-downloads help text (apple#1725)

Remove manually specified default value from help string since
ArgumentParser already appends it automatically from the property's
default value.

Signed-off-by: Charlie Le <charlie_le@apple.com>

Author: CharlieTLe

Sources/Services/ContainerAPIService/Client/Flags.swift

@@ -389,7 +389,7 @@ public struct Flags {
             self.maxConcurrentDownloads = maxConcurrentDownloads
         }
 
-        @Option(name: .long, help: "Maximum number of concurrent downloads (default: 3)")
+        @Option(name: .long, help: "Maximum number of concurrent downloads")
         public var maxConcurrentDownloads: Int = 3
     }
 }