docs: fix invalid link to platforms in README #914

Job	Run time
netlify_dryrun	2m 57s
just clippy	4m 48s
just check	4m 38s
docs	2m 29s
rustfmt	22s
cargo miri test --package forth3 (host)	1m 26s
just test	2m 22s
cargo build mnemos-d1	1m 41s
cargo build mnemos-esp32c3-buddy	1m 1s
cargo build mnemos-x86_64-bootloader	2m 27s
	24m 11s

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