ˆ (U+02C6) is a circumflex accent, not an exponentiation operator.
In the rest of the source code for this package, exponentation is
written as **, so do the same here.

Change-Id: I107b85be242ab79d152eb8a6fcf3ca2b197d7658
Reviewed-on: https://go-review.googlesource.com/17671Reviewed-by: Robert Griesemer <gri@golang.org>