runtime/trace: reword the trace package summary

Reword the package summary to be a little easier to read.

Change-Id: I84a9301a02e228b46165410a429548b3774762d5
GitHub-Last-Rev: 1342c7219f0ed06144d3bca9acd77790707e4b77
GitHub-Pull-Request: golang/go#26052
Reviewed-on: https://go-review.googlesource.com/120795Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/runtime/trace/trace.go

@@ -2,8 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// Package trace contains facilities for programs to generate trace
-// for Go execution tracer.
+// Package trace contains facilities for programs to generate traces
+// for the Go execution tracer.
 //
 // Tracing runtime activities
 //