cmd/covdata: format package comment for 'go doc' rendering

cmd/covdata: format package comment for 'go doc' rendering

Due to an errant newline, both 'go doc' and pkg.go.dev currently
interpret the long comment in cmd/covdata/doc.go as a file comment
instead of package documentation.

Removing the errant newline caused 'go doc' to render the comment, but
it does not strip out the interior '//' tokens from the '/* … */'
block.

Removing those tokens and fixing up indentation seems to give
satisfactory rendering.

Change-Id: I5757c649e7380b026f7d8d1b6fd3cb6dddfb27ad
Reviewed-on: https://go-review.googlesource.com/c/go/+/509635
Reviewed-by: Than McIntosh <thanm@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Bryan Mills <bcmills@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>