doc: Center text on line in content shape example

[bjoluc]

It took me a while to figure out why the text in the content example

line((0, 0), (3, 1), name: "line")
content(
  ("line.start", 0.5, "line.end"),
  angle: "line.end",
  padding: .1,
  anchor: "south",
  [Text on a line]
)

was not centered above the line. I suppose centering is a very common use case (maybe even intended in the example?), so I'd suggest using 50% here to save folks some time figuring out that 0.5 is not a percentage but an absolute length 🙂

[johannes-wolf]

Thank you! In older versions of cetz, floats between 0.0 and 1.0 were used for interpolation instead of ratios. The example has not been updated since.