gotest: fix the glob pattern in the top-of-file comment

R=golang-dev, adg
CC=golang-dev
https://golang.org/cl/1847042

src/cmd/gotest/gotest

@@ -3,10 +3,10 @@
 # Use of this source code is governed by a BSD-style
 # license that can be found in the LICENSE file.
 
-# Using all the test*.go files in the current directory, write out a file
+# Using all the *_test.go files in the current directory, write out a file
 # _testmain.go that runs all its tests. Compile everything and run the
 # tests.
-# If files are named on the command line, use them instead of test*.go.
+# If files are named on the command line, use them instead of *_test.go.
 
 # Makes egrep,grep work better in general if we put them
 # in ordinary C mode instead of what the current language is.