RFR: 8320526: Use title case in building.md

[Iris Clark]

On Tue, 21 Nov 2023 15:58:06 GMT, Magnus Ihse Bursie <ihse at openjdk.org> wrote:

> We should use title case consistently in building.md.
> 
> I thought I was done with fixing up the build README, but then I noticed this and I couldn't unsee it...

Marked as reviewed by iris (Reviewer).

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PR Review: https://git.openjdk.org/jdk/pull/16765#pullrequestreview-1742444708