buildlet: fix doc typo
buildlet: fix doc typo

Change-Id: I2e43aa33634eb63b7cbdcc6ba6296fe8e857ad81
Reviewed-on: https://go-review.googlesource.com/36912
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

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