Fix typo: withing -> within

[Neob91]

Love the library, thanks!

[codecov[bot]]

Codecov Report

Patch and project coverage have no change.

Comparison is base (941f7c1) 95.38% compared to head (9881098) 95.38%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #257 +/- ## ======================================= Coverage 95.38% 95.38% ======================================= Files 9 9 Lines 1018 1018 Branches 215 215 ======================================= Hits 971 971 Misses 43 43 Partials 4 4 ``` | [Impacted Files](https://app.codecov.io/gh/jquast/blessed/pull/257?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jeff+Quast) | Coverage Δ | | |---|---|---| | [blessed/terminal.py](https://app.codecov.io/gh/jquast/blessed/pull/257?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jeff+Quast#diff-Ymxlc3NlZC90ZXJtaW5hbC5weQ==) | `98.27% <ø> (ø)` | |

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[avylove]

Thanks for the PR!