os: fix grammar in ProcessState comment

Asymptotic convergence.

R=golang-dev, dsymonds
CC=golang-dev
https://golang.org/cl/5719045

src/pkg/os/exec_plan9.go

@@ -101,7 +101,7 @@ func findProcess(pid int) (p *Process, err error) {
 	return newProcess(pid, 0), nil
 }
 
-// ProcessState stores information about process as reported by Wait.
+// ProcessState stores information about a process, as reported by Wait.
 type ProcessState struct {
 	pid    int              // The process's id.
 	status *syscall.Waitmsg // System-dependent status info.

src/pkg/os/exec_posix.go

@@ -44,7 +44,7 @@ func (p *Process) kill() error {
 	return p.Signal(Kill)
 }
 
-// ProcessState stores information about process as reported by Wait.
+// ProcessState stores information about a process, as reported by Wait.
 type ProcessState struct {
 	pid    int                // The process's id.
 	status syscall.WaitStatus // System-dependent status info.