trace: fix minor typo
Fixes
// ...
// childSpan.Finish(trace.WithResponse(resp))
//
-// When a span created by SpanFromRequest or SpamFromHeader is finished, the
+// When a span created by SpanFromRequest or SpanFromHeader is finished, the
// finished spans in the corresponding trace -- the span itself and its
// descendants -- are uploaded to the Stackdriver Trace server using the
// *Client that created the span. Finish returns immediately, and uploading
