Fix readtable name in readme: lol package to keyword

Since recent commit 9253ec0, the invocation in README.md

```
(named-readtables:in-readtable lol:lol-syntax)
```
no longer works - the lol-syntax symbol is now in keywords package (i.e., `:lol-syntax`).

If this is intentional change, README should be updated. Note that it breaks dependent packages that use it ([one in quicklisp](http://report.quicklisp.org/2023-12-01/failure-report.html)).