Replace bitwise operation with direct assignment when capping allowance to max value.

The previous implementation used addition with bitwise NOT operator:
rl.allowance += ^((current - rl.max) - 1)

This is functionally equivalent to:
rl.allowance = rl.max

Benefits of this change:

• Improved code readability and maintainability
• Better performance by avoiding unnecessary bitwise operations
• Clearer intent: the goal is to cap allowance at maximum value
• No functional changes to the rate limiting behavior

The bitwise approach was likely inherited from the original Stack Overflow
algorithm, but direct assignment is more idiomatic in Go for this use case.