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 <[email protected]>
Run-TryBot: Hyang-Ah Hana Kim <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>

src/runtime/debug/mod.go

@@ -22,7 +22,7 @@ func ReadBuildInfo() (info *BuildInfo, ok bool) {
 // the running binary.
 type BuildInfo struct {
 	Path string    // The main package path
-	Main Module    // The main module information
+	Main Module    // The module containing the main package
 	Deps []*Module // Module dependencies
 }