runtime: fix typo in error message
runtime: fix typo in error message

LGTM=robert.hencke, iant
R=golang-codereviews, robert.hencke, iant
CC=golang-codereviews
https://codereview.appspot.com/89760043

Committer: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang