Fix typo in Pcg.zig's fill function
Fix typo in Pcg.zig's fill function

When filling the last (len % 4) bytes of a buffer, the random number n was only being shifted right by 4 bits for each byte instead of 8. A random u16, for example, would always have its middle two nybbles be equal when generated this way. For comparison, Isaac64.zig, Sfc64.zig, and Xoroshiro128.zig all correctly shift right by 8 bits for each of the last bytes in their nearly identical fill functions.
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