[Docs] Fix typo in docs for start and end of century

[pauln-relay]

Pull Request Check List

• Added tests for changed code.
• Updated documentation for changed code.

Summary

• Fix typo in docs for dt.start_of('century') and dt.end_of('century') to reflect correct century bounds
• No code change - validated the rest of the examples for modifiers ✅

Validation

>>> import pendulum
>>> dt = pendulum.datetime(2012, 1, 31, 12, 0, 0)
>>> dt.start_of('century')
DateTime(2001, 1, 1, 0, 0, 0, tzinfo=Timezone('UTC'))
>>> dt.end_of('century')
DateTime(2100, 12, 31, 23, 59, 59, 999999, tzinfo=Timezone('UTC'))

[ashb]

Not the values I would have expected, but this has been consistent since Pendulm 1.0.2.

[codspeed-hq]

CodSpeed Performance Report

Merging #910 will not alter performance

_{Comparing pauln-relay:docs-fix-start-of-century (881bd7f) with master (2adcc02)}

Summary

✅ 1 untouched benchmarks

[pauln-relay]

Not the values I would have expected, but this has been consistent since Pendulm 1.0.2.

Same - I learned more about centuries today than I had planned