qdoc: Correct a misuse of a local variable

This update fixes a bug caused by reusing an index varible
name declared in an outer loop in an inner loop, which made
qdoc crash in one case.

Change-Id: I27abb4ea3241be620c8c226732e83ef42b575da9
Reviewed-by: Topi Reiniö <topi.reinio@qt.io>
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@qt.io>