fix(color): off-by-one for INDEXED_COLORS access

[RoloEdits]

Current implimentation sets the value right from the color tags value, e.g <color indexed="2", which is already zero-indexed.

umya-spreadsheet/src/structs/color.rs

Lines 212 to 215 in 3c34cb8

	Ok(ref attr) if attr.key.0 == b"indexed" => {
	self.indexed
	.set_value_string(get_attribute_value(attr).unwrap());
	}

The - 1 would then access the previous indexes color if it was <color indexed="1..", or return None if it was <color indexed="0".

The flipside to this is change is that the implimentation for set_value no longer needs to add 1. The tests for this have been modified for this change.

I added a single test with red color "RED" in A1

which results in an styles.xml with:

<font>
    <sz val="11.000000" />
    <color indexed="2" />
    <name val="Calibri" />
    <scheme val="minor" />
</font>

[MathNya]

@RoloEdits
Thank you very much.
We will merge three PRs.