Replace the only two occurrences of \t in C code

[dtolnay]

The use of hard tabs, where everything else in this project uses 4-space indent, causes the code to be displayed in a misleading way when tab size is anything other than 8, including in GitHub's web UI.

https://github.com/yaml/libyaml/blob/f8f760f7387d2cc56a2fc7b1be313a3bf3f7f58c/src/api.c#L123-L126

[dtolnay]

CI failure appears unrelated.

Using default tag: latest
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