fix: Fix missing Result handling in getrandom registration example

[voronor]

issue in the Providing Randomness section of the documentation. The example code for registering a custom RNG handler using getrandom::register_custom_getrandom! is missing the proper handling of the Result returned by this macro.

the line:

getrandom::register_custom_getrandom!(custom_getrandom);

should be updated to:

getrandom::register_custom_getrandom!(custom_getrandom)?;

this ensures that any potential errors are properly propagated.
i've corrected this to align with best practices and avoid confusion for developers integrating this into their code.

Thanks for maintaining this resource—it’s incredibly helpful! Let me know if you’d like further adjustments.

[ma2bd]

Thanks. Let me check. This is related to #184

[ma2bd]

I don't see the issue: https://docs.rs/getrandom/latest/getrandom/macro.register_custom_getrandom.html