draw: fix Transformer documentation formatting

Each span of unindented non-blank lines is converted into a
single paragraph. This isn't desired here. Indent the matrix,
so that it's converted into a <pre> block. That also prevents
the previous line from being interpreted as a heading.

Reference: https://godoc.org/go/doc#ToHTML

Fixes golang/go#28683

Change-Id: Ibc5488d5cc66fe3a5f2bbe2fe23628dd08276037
Reviewed-on: https://go-review.googlesource.com/c/148573
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
diff --git a/draw/scale.go b/draw/scale.go
index 3fc30c4..11784d4 100644
--- a/draw/scale.go
+++ b/draw/scale.go
@@ -46,8 +46,8 @@
 // For example, if m is the matrix
-// m00 m01 m02
-// m10 m11 m12
+// 	m00 m01 m02
+// 	m10 m11 m12
 // then the src-space point (sx, sy) maps to the dst-space point
 // (m00*sx + m01*sy + m02, m10*sx + m11*sy + m12).