Correct documentation.

[veelo]

Without the cast recent compilers complain with

Error: none of the overloads of `sharedLog` are callable using argument types `(FileLogger)`
        Candidates are: `std.logger.core.sharedLog()`
                        `std.logger.core.sharedLog(shared(Logger) logger)`

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	23487	regression	std.experimental.logger assigning FileLogger to sharedLog no longer works

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "stable + phobos#8995"

[veelo]

The shared cheats have been moved into the logger implementation by Átila, so that user code can follow proper shared practices, which is now documented.

I noted that stable has entered beta state, let me know if I should rather target master.