A cross C compiler should be specified when cross building go
for android on x86 host. This CL fixes the typo in README.

Change-Id: Ideda915364d9f7f0fcd53146abc1e74506a8a8e6
Reviewed-on: https://go-review.googlesource.com/c/go/+/177777Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>
Reviewed-by: Elias Naur <mail@eliasnaur.com>