doc: update Wasmtime flags

Wasmtime's `--wasm-features` and `--wasi-modules` flags have been renamed
since these docs were initially written.

Additionally, from my testing I don't believe `--wasm threads` is needed if
`--wasi threads` is passed already.

Author: DaniPopes

Diff for: src/doc/rustc/src/platform-support/wasm32-wasip1-threads.md

@@ -100,7 +100,7 @@ This target is not a stable target. This means that there are a few engines
 which implement the `wasi-threads` feature and if they do they're likely behind a
 flag, for example:
 
-* Wasmtime - `--wasm-features=threads --wasi-modules=experimental-wasi-threads`
+* Wasmtime - `--wasi threads`
 * [WAMR](https://github.com/bytecodealliance/wasm-micro-runtime) - needs to be built with WAMR_BUILD_LIB_WASI_THREADS=1
 
 ## Building the target

Diff for: src/doc/rustc/src/platform-support/wasm64-unknown-unknown.md

@@ -36,7 +36,7 @@ which implement the `memory64` feature and if they do they're likely behind a
 flag, for example:
 
 * Nodejs - `--experimental-wasm-memory64`
-* Wasmtime - `--wasm-features memory64`
+* Wasmtime - `--wasm memory64`
 
 Also note that at this time the `wasm64-unknown-unknown` target assumes the
 presence of other merged wasm proposals such as (with their LLVM feature flags):