[binder]: Correct the index for getReturnString

The cmd code can not as the index drectly.

Signed-off-by: songjinshi <songjinshi@xiaomi.com>
(cherry picked from commit 73a7dde7c3e5c8437f396deeb200f250b3346be7)

Bug: 129785390
Test: N/A
Change-Id: Icddad46faef7a77acbd7edf118c081ac10501f47
1 file changed