design: contracts: typo fix in example.
design: contracts: typo fix in example.

Change-Id: I73bce7bd3603a84cc539de0135fcefa4e45253ca
Reviewed-on: https://go-review.googlesource.com/132795
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/proposal,golang/proposal