os/exec: Examples with missing imports

[gopherbot]

by toqueteos:

Example on: http://golang.org/pkg/os/exec/#example_Cmd_Output
Requires: "os/exec"

Example on: http://golang.org/pkg/os/exec/#example_Cmd_StdoutPipe
Requires: "encoding/json"

Only docs from http://golang.org are affected, tip has no playground integration on
those examples.

[rsc]

Comment 1:

Labels changed: added priority-later, removed priority-triage.

Owner changed to @adg.

Status changed to Accepted.

[adg]

Comment 2:

Status changed to Started.

[adg]

Comment 3:

https://golang.org/cl/7067048/

[adg]

Comment 4:

This issue was closed by revision 60544b6.

Status changed to Fixed.