Update origin-start-panic-abort's README.md.

Now that origin uses `cfg(panic = "unwind")` it no longer has an
"unwinding" feature.

Author: sunfishcode

example-crates/origin-start-panic-abort/README.md

@@ -1,4 +1,3 @@
-Like the [origin-start example], but uses panic=abort in Cargo.toml instead of
-enabling origin's "unwinding" feature.
+Like the [origin-start example], but uses panic=abort in Cargo.toml.
 
 [origin-start example]: https://github.com/sunfishcode/origin/blob/main/example-crates/origin-start/README.md