This change renames the example to follow correct naming convention,
as documented at https://godoc.org/testing#hdr-Examples. As a result,
it shows up in godoc.
This issue was caught and reported by vet:
$ go vet golang.org/x/image/font/sfnt
# golang.org/x/image/font/sfnt_test
font/sfnt/example_test.go:19: ExampleRasterizeGlyph refers to unknown identifier: RasterizeGlyph
Fixesgolang/go#28684
Change-Id: I2749c638c5f3ed15e4db0448bc7a5e2c12c056e5
Reviewed-on: https://go-review.googlesource.com/c/148576
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>