misc/ios: fix wording for command line instructions

A typo was made, which I noticed while looking through the recent master
commits.

Change-Id: Ieed5d6664a1f3ff5892d59abf194963b44ef0e55
Reviewed-on: https://go-review.googlesource.com/c/go/+/280454


Trust: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Cherry Zhang <cherryyz@google.com>