message: tweak isolation-error message in README
diff: @@ -221,11 +221,11 @@ environment variable:
  systems, and randomness.
  * `-Zmiri-isolation-error=<action>` configures Miri's response to operations
  requiring host access while isolation is enabled. `abort`, `hide`, `warn`,
  - and `warn-nobacktrace` are the supported actions. Default action is `abort`
  - which halts the machine. Rest of the actions configure it to return an error
  - code for the op and continue executing. `warn` prints backtrace that could
  - be used to trace the call. `warn-nobacktrace` is less verbose without
  - backtrace. `hide` hides the warning.
  + and `warn-nobacktrace` are the supported actions. The default is to `abort`,
  + which halts the machine. Some (but not all) operations also support continuing
  + execution with a "permission denied" error being returned to the program.
  + `warn` prints a full backtrace when that happen; `warn-nobacktrace` is less
  + verbose. `hide` hides the warning entirely.
  * `-Zmiri-env-exclude=<var>` keeps the `var` environment variable isolated from
  the host so that it cannot be accessed by the program. Can be used multiple
  times to exclude several variables. On Windows, the `TERM` environment
  