Correct the capitalisation of the `png` tag #1764

Job	Run time
cargo check (msrv) (wasm32)	39s
cargo check (aarch64-android)	19s
cargo clippy (windows-latest)	2m 9s
cargo check (msrv) (windows-latest)	3m 10s
cargo clippy (macos-latest)	1m 32s
cargo clippy (wasm32)	1m 36s
cargo check (msrv) (macos-latest)	36s
cargo clippy (ubuntu-latest)	1m 42s
cargo doc	1m 48s
cargo check (msrv) (ubuntu-latest)	47s
formatting	25s
Prime LFS Cache	7s
cargo test (wasm32)	41s
typos	4s
cargo test (windows-latest)	2m 56s
cargo test (macos-latest)	1m 42s
cargo test (ubuntu-latest)	4m 5s
	24m 18s

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