content: fix grammar in nil maps slide
content: fix grammar in nil maps slide

Change-Id: I7f4bf5ad97aed66b59ee9087241a71e3beee40cb
Reviewed-on: https://go-review.googlesource.com/21590
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

golians/tour,dupoxy/go-tour-fr,golang/tour,scryner/tour,dupoxy/go-tour-fr,golians/tour,scryner/tour,Go-zh/tour,shuLhan/go-tour-id,shuLhan/tour,shuLhan/go-tour-id,Go-zh/tour,shuLhan/tour,scryner/tour,shuLhan/tour,zorion/go-tour-ca,scryner/tour,Go-zh/tour,zorion/go-tour-ca,Go-zh/tour,dupoxy/go-tour-fr,shuLhan/go-tour-id,golians/tour