doc/go1.5.html: refer to ppc64 as 64-bit PowerPC, not Power 64

Saying "Power 64" was wrong for reasons I don't remember.
(Those reasons are why we stopped using GOARCH=power64.)

Change-Id: Ifaac78d5733bfc780df01b1a66da766af0b17726
Reviewed-on: https://go-review.googlesource.com/13675


Reviewed-by: Rob Pike <r@golang.org>