Minor: "cargo doc" - Fixed lint issues with ContourBuilder.

[martinfrances107]

I love this crate.... it is really useful.

I was reading the docs and notice a tiny blemish

running

cargo doc

Showed that the flaw was repeated 3 times, thankfully the fix is just a minor nudge.

In temrs of "self" review .. I can state that I have visually checked the doc after the change and the fix works.

warning: unresolved link to `ContourBuilder`
 --> src/line.rs:4:73
  |
4 | /// A line has the geometry and threshold of a contour ring, built by [`ContourBuilder`].
  |                                                                         ^^^^^^^^^^^^^^ no item named `ContourBuilder` in scope
  |
  = help: to escape `[` and `]` characters, add '\' before them like `\[` or `\]`

warning: `contour` (lib doc) generated 3 warnings

[mthh]

Thank you!