bufio: Fix typo in scan.go documentation

Apologies for the the nitpicky PR. I believe there is a minor typo in the documentation of `MaxScanTokenSize`, which confused me for a moment when I went to search for the referenced method, `Scan.Buffer`. Thanks!

Change-Id: I5d21e77276285206497fe75291001032c255cace
GitHub-Last-Rev: 635e35c0191c11f2b6966b5e58cf91a1064099da
GitHub-Pull-Request: golang/go#32193
Reviewed-on: https://go-review.googlesource.com/c/go/+/178637Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/bufio/scan.go

@@ -73,7 +73,7 @@ var (
 
 const (
 	// MaxScanTokenSize is the maximum size used to buffer a token
-	// unless the user provides an explicit buffer with Scan.Buffer.
+	// unless the user provides an explicit buffer with Scanner.Buffer.
 	// The actual maximum token size may be smaller as the buffer
 	// may need to include, for instance, a newline.
 	MaxScanTokenSize = 64 * 1024