README: the versions it names are the versions that exist

Every README here opens by showing what a program writes in its manifest, which
is the first thing a reader copies and the last thing anyone edits. These lines
had drifted --- the specification's own README asked for a version four minor
releases old --- and nothing checked them.

`openkal/tools/check-readme-versions.sh` now does.

Author: speak-agent

README.md

@@ -6,7 +6,7 @@ C library.
 
 ```toml
 [dependencies]
-openkal-llvm-runtime = "0.3.1"
+openkal-llvm-runtime = "0.4.0"
 ```
 
 A C++ standard library is not portable in the way a program is. It is