polish the doc style in `MicrovmImageS3Fetcher`
polish the doc style in `MicrovmImageS3Fetcher`

Signed-off-by: Radu Weiss <e20b2d982c117e9298ee475ba7241e3f2127299b@amazon.com>

firecracker-microvm/firecracker,firecracker-microvm/firecracker,firecracker-microvm/firecracker,firecracker-microvm/firecracker,firecracker-microvm/firecracker