go/build: remove stale reference to ioutil.ReadDir

The package moved away from the deprecated ioutil API some time ago.

Change-Id: Iecb1baa9285af1f721a04eb40f8dafdd72474c4d
Reviewed-on: https://go-review.googlesource.com/c/go/+/438515
Reviewed-by: Bryan Mills <[email protected]>
Run-TryBot: Daniel Martí <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>

src/go/build/build.go

@@ -103,7 +103,7 @@ type Context struct {
 
 	// ReadDir returns a slice of fs.FileInfo, sorted by Name,
 	// describing the content of the named directory.
-	// If ReadDir is nil, Import uses ioutil.ReadDir.
+	// If ReadDir is nil, Import uses os.ReadDir.
 	ReadDir func(dir string) ([]fs.FileInfo, error)
 
 	// OpenFile opens a file (not a directory) for reading.