trace: fix minor typo

Fixes #722.

Change-Id: I5abcbac84d6642d8bda3a93a31cec67f7e4730f7
Reviewed-on: https://code-review.googlesource.com/15870
Reviewed-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Ross Light <light@google.com>
Reviewed-by: Michael Darakananda <pongad@google.com>

trace/trace.go

@@ -113,7 +113,7 @@
 //   ...
 //   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