embedded-hal-async: fix missing link in README

[newAM]

Restored to what it was in bc3fc45.

[rust-highfive]

r? @therealprof

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[eldruin]

Nice catch, thanks!
bors r+

[bors]

Build succeeded:

• clippy
• fmt
• test (1.54.0, thumbv6m-none-eabi)
• test (1.54.0, thumbv7m-none-eabi)
• test (1.54.0, x86_64-unknown-linux-gnu)
• test (nightly, thumbv6m-none-eabi)
• test (nightly, thumbv7m-none-eabi)
• test (nightly, x86_64-unknown-linux-gnu)
• test (stable, thumbv6m-none-eabi)
• test (stable, thumbv7m-none-eabi)
• test (stable, x86_64-unknown-linux-gnu)