runtime/debug: add missing period

Pointed out in review of CL 453602,
but it looks like I forgot to re-upload before submitting.

Change-Id: I8f4fac52ea0f904f6f9b06e13fc8ed2e778f2360
Reviewed-on: https://go-review.googlesource.com/c/go/+/454835
TryBot-Result: Gopher Robot <[email protected]>
Run-TryBot: Russ Cox <[email protected]>
Auto-Submit: Russ Cox <[email protected]>
Reviewed-by: Julie Qiu <[email protected]>

Author: rsc

src/runtime/debug/mod.go

@@ -47,7 +47,7 @@ type BuildInfo struct {
 	// (for example, "golang.org/x/tools/cmd/stringer").
 	Path string
 
-	// Main describes the module that contains the main package for the binary
+	// Main describes the module that contains the main package for the binary.
 	Main Module
 
 	// Deps describes all the dependency modules, both direct and indirect,