Fix doc index name on Windows

[Jonathan Larmour]

Eli Zaretskii wrote:
> 
> I don't like the name you suggest, "GDB Index".  That chapter is not
> an index of GDB, it's an index of a manual.

I was just going along with how other GNU projects have dealt with the 
issue. I have no strong feelings on this.

>  I can go with something
> like "Concept and Command Index" instead (and will accept such a patch
> if you insist on fixing this in GDB),

I'm happy to go along with whatever you suggest as the alternative.

Jifl
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