fix typo in `Attachment` docs (#2885) #907

Job	Run time
Test (stable)	3m 13s
Test (beta)	3m 34s
Test (Windows)	6m 41s
Build docs	46s
Test (native-tls)	2m 58s
Test (no default features)	1m 6s
Examples	4m 4s
Test (no cache)	3m 5s
Test (no gateway)	1m 52s
Test (unstable Discord API features)	3m 18s
Test (macOS)	1m 52s
Test (simd-json)	3m 14s
Test (chrono, chrono)	50s
Test (unstable API + typesize, default unstable_discord_api typesize, true)	1m 22s
Check minimal versions	1m 7s
Test (builder without model, builder, true)	33s
Test (unstable Discord API (no default features), unstable_discord_api, true)	33s
Test (nightly)	3m 37s
MSRV	1m 5s
	44m 50s

Provide feedback