Relaxed overly tight floating-point comparison

This fixes a test failure on arm64 Macs, with default compiler options (that is, without `-ffp-model=strict`), where the dist2 value is not exactly zero but very tiny (due to fma, fused multiply add).

Specifically, in the UnitTestCells test it previously output:

```
Testing EvaluatePosition in/out test...cell center failed inOut: 1 point: -1.04083e-17, 0, 0-> pcoords: 0, 0, 0: closestPoint: -1.04083e-17, 0, 0 dist2: 1.08334e-34 weights: -0.0625 -0.0625 0.5625 0.5625
vtkCubicLine FAILED
```

Also fixed unrelated spelling/typo.

Co-authored-by: David Gobbi <david.gobbi@gmail.com>