lib/time: fix RFC 6557 url

Change-Id: I59406ee7dbab7b2a0404b62061af552b6b4ecf5f
GitHub-Last-Rev: 7cad5ae
GitHub-Pull-Request: #47696
Reviewed-on: https://go-review.googlesource.com/c/go/+/342209
Reviewed-by: Emmanuel Odeke <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>
Run-TryBot: Emmanuel Odeke <[email protected]>
TryBot-Result: Go Bot <[email protected]>

Author: 180909

lib/time/README

@@ -5,6 +5,6 @@ The IANA asserts that the database is in the public domain.
 For more information, see
 https://www.iana.org/time-zones
 ftp://ftp.iana.org/tz/code/tz-link.html
-http://tools.ietf.org/html/rfc6557
+https://datatracker.ietf.org/doc/html/rfc6557
 
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