x/text/currency: fix date format on example

[navossoc]

What version of Go are you using (go version)?

go version go1.9.2 windows/amd64

Does this issue reproduce with the latest release?

Yes

What operating system and processor architecture are you using (go env)?

set GOARCH=amd64
set GOBIN=
set GOEXE=.exe
set GOHOSTARCH=amd64
set GOHOSTOS=windows
set GOOS=windows
set GOPATH=C:\Users\Rafael\go
set GORACE=
set GOROOT=C:\Go
set GOTOOLDIR=C:\Go\pkg\tool\windows_amd64
set GCCGO=gccgo
set CC=gcc
set GOGCCFLAGS=-m64 -mthreads -fmessage-length=0
set CXX=g++
set CGO_ENABLED=1
set CGO_CFLAGS=-g -O2
set CGO_CPPFLAGS=
set CGO_CXXFLAGS=-g -O2
set CGO_FFLAGS=-g -O2
set CGO_LDFLAGS=-g -O2
set PKG_CONFIG=pkg-config

What did you do?

I've created a pull request, but it takes a lot of work to become a contributor to change only two numbers.
There is more info here: golang/text#3

What did you expect to see?

// GBP is used in GB since: 1694-07-27
// GIP is used in GI since: 1713-01-01
// USD is used in US since: 1792-01-01

What did you see instead?

// GBP is used in GB since: 1694-07-07
// GIP is used in GI since: 1713-01-01
// USD is used in US since: 1792-01-01

[gopherbot]

Change https://golang.org/cl/85398 mentions this issue: currency: fix format in example