cmd/compile: tweak example command in README
While running ./src/all.bash, I got the following error:
all.bash must be run from $GOROOT/src
Change-Id: Iad271654ff8647501a324c282f01baf39605e3df
GitHub-Last-Rev:
2e289babf1bc81c1a44c9a72a29659d8bf9d827b
GitHub-Pull-Request: golang/go#75050
Reviewed-on: https://go-review.googlesource.com/c/go/+/696296
Reviewed-by: t hepudds <thepudds1460@gmail.com>
Reviewed-by: Keith Randall <khr@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>