x/website: doc/go1.21: inconsistent amd64 capitalization

[willfaught]

On amd64, frameless nosplit assembly functions are [...]

On Windows AMD64, the linker [...]

[heschi]

Sounds like compiler stuff. cc @golang/compiler

[gopherbot]

Change https://go.dev/cl/510295 mentions this issue: doc/go1.21: use consistent capitalization for amd64