Update wgpu/src/api/blas.rs with better docs

Co-authored-by: Connor Fitzgerald <connorwadefitzgerald@gmail.com>

wgpu/src/api/blas.rs

@@ -102,7 +102,7 @@ pub struct BlasTriangleGeometry<'a> {
     pub vertex_stride: wgt::BufferAddress,
     /// Index buffer (optional).
     pub index_buffer: Option<&'a Buffer>,
-    /// Index buffer offset as a factor of the size of the index type (optional, required if index buffer is present).
+    /// Number of indexes to skip in the index buffer (optional, required if index buffer is present).
     pub first_index: Option<u32>,
     /// Transform buffer containing 3x4 (rows x columns, row major) affine transform matrices `[f32; 12]` (optional).
     pub transform_buffer: Option<&'a Buffer>,