internal/chartconfig: consolidate indentation in config.txt

Some of the lines were indented by 2 spaces, some by 4. Make them all
indented by a single tab, and the viewers can use their preferred tab
width in their text editor. This is based on gofmt using a single tab
for indentation (spaces are used for alignment within the rest of the
line, across adjacent lines that are equally indented).

Generating the ./internal/configgen package before and after this has
the same results.

Change-Id: I051770ef42dbe2324a66eb41df4dfbaccd117a27
Reviewed-on: https://go-review.googlesource.com/c/telemetry/+/783260
Auto-Submit: Hongxiang Jiang <hxjiang@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Hongxiang Jiang <hxjiang@golang.org>
Commit-Queue: Hongxiang Jiang <hxjiang@golang.org>