content: add link to Polish translation
content: add link to Polish translation

Change-Id: I8afdf85924e8eeba179d23cc9f978d4236ab740e
GitHub-Last-Rev: 9ad40ae1da2fb486932e777b02db5cf57fc58320
GitHub-Pull-Request: golang/tour#1136
Reviewed-on: https://go-review.googlesource.com/c/tour/+/298150
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>
Run-TryBot: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Trust: Alberto Donizetti <90b16103a35511ced020676f3544f17117fb4c92@gmail.com>

golang/tour