simplify Qemu instructions

Turns out we don't need a special case for KVM.
  https://qemu.readthedocs.io/en/latest/system/invocation.html

shell/README.md

@@ -41,25 +41,23 @@ Currently runs a tiny dialect of Lisp. Steps to run it from the top-level:
   results. Hit `ctrl-m` to focus on the `...` after a run, and browse how the
   _trace_ of how the results were computed. [Here's a 2-minute demo](https://archive.org/details/akkartik-mu-2021-05-31).
 
-3. If your Qemu installation supports them, one of these commandline arguments
-   may speed up emulation:
+3. If your Qemu installation supports them, an `-accel` arguments
+   will speed up emulation. Try `-accel help` to list your options.
 
-  - `-enable-kvm`
-  - `-accel ___` (run with `-accel help` for a list of available options)
+  Once you select an accelerator, I recommend also adjusting the `responsiveness`
+  mask in shell/evaluate.mu, which controls how frequently the fake screen
+  updates within the REPL. Smaller values will seem more responsive, larger
+  values will leave more time to run your programs. I like to see the screen
+  update about once a second. Some suggested values depending on how fast your
+  Qemu is running:
 
-  If you do this, I recommend also adjusting the `responsiveness` mask in
-  shell/evaluate.mu, which controls how frequently the fake screen updates.
-  Smaller values will seem more responsive, larger values will leave more time
-  to run your programs. I like to see the screen update about once a second.
-  Some suggested values depending on how fast your Qemu is running:
-
-  - `-enable-kvm` on a T420s running Linux: `0xffff/responsiveness=64k`
+  - `-accel kvm` on a T420s running Linux: `0xffff/responsiveness=64k`
   - `-accel tcg` on a 2019 Mac: `0xfff/responsiveness=4k`
 
   Putting it all together, here's the command I typically use on Linux:
 
   ```
-  qemu-system-i386 -m 2G -enable-kvm -hda code.img -hdb data.img
+  qemu-system-i386 -m 2G -accel kvm -hda code.img -hdb data.img
   ```
 
 ### Indent-sensitivity