[x/tour] content: explain why Big cannot be cast to an int
[x/tour] content: explain why Big cannot be cast to an int

The error message says that Big overflows an int.
This additional description points towards the maximum
size of an int.

It does not go into detail about int sometimes being either
32 or 64 bits wide depending on the platform, since that was
mentioned on an earlier slide.

Change-Id: Id0d1781789a875f6bd90d0062f70bfb432620850
Reviewed-on: https://go-review.googlesource.com/15600
Reviewed-by: Andrew Gerrand <395a7d33bec8475c9b83b7d440f141bcbd994aa5@golang.org>
X-Tour-Commit: 459d92e21d28a6d9189af4ce917c4e92803ca357

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