GD: Update documentation with correct image format

Per Gitiles documentation, only PNG, JPEG, and GIF formats
are supported when images are linked from a GIT repo

See https://gerrit.googlesource.com/gitiles/+/HEAD/Documentation/markdown.md#images

Bug: 155227498
Test: view markdown result
Change-Id: I2c6f73bb221d8e632c0c0407c77e7d4c800711ec