os: fix grammar nit

There should not be a comma before "and" in the original text,
because what follows is not a complete sentence. Rewrite.

Change-Id: Ie99f204cc87e911fb46149e2eb65e132fa1eb63a
Reviewed-on: https://go-review.googlesource.com/87020
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/os/exec.go

@@ -86,7 +86,7 @@ func FindProcess(pid int) (*Process, error) {
 
 // StartProcess starts a new process with the program, arguments and attributes
 // specified by name, argv and attr. The argv slice will become os.Args in the
-// new process, and normally starts with the program name.
+// new process, so it normally starts with the program name.
 //
 // StartProcess is a low-level interface. The os/exec package provides
 // higher-level interfaces.