runtime/debug: correct BuildInfo.Main documentation

runtime/debug: correct BuildInfo.Main documentation

The term "main module" has a special meaning [1]
and is not what we intended to refer to with BuildInfo.Main.

[1] https://golang.org/cmd/go/#hdr-The_main_module_and_the_build_list

Updates #33975

Change-Id: Ieaba5fcacee2e87c5c15fa7425527bbd64ada5d5
Reviewed-on: https://go-review.googlesource.com/c/go/+/196522
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Hyang-Ah Hana Kim <hyangah@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>