cmd/pushback: fix placeholder PR numbers
cmd/pushback: fix placeholder PR numbers

golang/go placeholder had to be changed to 19117 given the previous
placeholder cannot be reopened.

golang/oauth2 is using the correct PR number now.

Change-Id: I9ee4801d6b4abc340fa1c9a74e3c0a6649f537f0
Reviewed-on: https://go-review.googlesource.com/37108
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/build,golang/build,golang/build,golang/build