errors: fix comment referencing the Wrapper interface

The Unwrap function performs a type assertion looking for the Wrapper
interface. The method of that interface is called Unwrap but the
interface itself is called Wrapper.

Change-Id: Ie3bf296f93b773d36015bcab2a0e6585d39783c7
GitHub-Last-Rev: 32b1a0c2f8bf8f3eaebf6de252571d82313e86e0
GitHub-Pull-Request: golang/go#31794
Reviewed-on: https://go-review.googlesource.com/c/go/+/174917Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/errors/wrap.go

@@ -34,7 +34,7 @@ func (e noWrapper) FormatError(p Printer) (next error) {
 }
 
 // Unwrap returns the result of calling the Unwrap method on err, if err
-// implements Unwrap. Otherwise, Unwrap returns nil.
+// implements Wrapper. Otherwise, Unwrap returns nil.
 func Unwrap(err error) error {
 	u, ok := err.(Wrapper)
 	if !ok {