doc/go1.21: consistently use spaces for indentation

doc/go1.21: consistently use spaces for indentation

Change-Id: Ib0aec9ee6cd7aae1821c82e67f0c67be01122992
Reviewed-on: https://go-review.googlesource.com/c/go/+/509677
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: David Chase <drchase@google.com>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>