Update README.md: add missing 'time' import in code example

[neuralcoral]

Description

This PR fixes an issue in the README.md code example where the time package is not imported. Without this import, users attempting to run the example code receive the following error:

./main.go:23:32: undefined: time

Changes

• Added the missing time import in the README.md example code.

Motivation

Adding the time import improves the accuracy of the README example, allowing users to copy and run the example without encountering errors.

[github-actions]

Benchmark Results

goos: linux
goarch: amd64
pkg: github.com/go-chi/httprate
cpu: AMD EPYC 7763 64-Core Processor                
               │ master.txt  │               pr.txt               │
               │   sec/op    │   sec/op     vs base               │
LocalCounter-4   19.40m ± 2%   19.67m ± 1%  +1.40% (p=0.019 n=10)

               │  master.txt  │             pr.txt             │
               │     B/op     │     B/op      vs base          │
LocalCounter-4   2.833Mi ± 0%   2.830Mi ± 0%  ~ (p=0.218 n=10)

               │ master.txt  │            pr.txt             │
               │  allocs/op  │  allocs/op   vs base          │
LocalCounter-4   121.5k ± 0%   121.4k ± 0%  ~ (p=0.184 n=10)

[VojtechVitek]

LGTM. Thank you