docs: fix typo in getting-started.md

[So-chiru]

Please check if the PR fulfills these requirements

• It's submitted to right branch according to our branching model
• It's right issue type on title
• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)
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Description

Fixed key typo of header field to height, since IMonthOptions.grid? should implements header?: { height?: number }

tui.calendar/index.d.ts

Line 162 in a1d5e1c

height?: number;

[seonim-ryu]

Can one of the admins verify this patch?

[adhrinae]

@So-chiru Thanks for your contribution :) Will merge this.