Remove unused parameter from copy_partition kernel documentation (#8283)

PR #8232 removed the `num_partitions` parameter from the `copy_partition` kernel but failed to update the associated documentation.

Authors:
  - Robert Maynard (https://github.com/robertmaynard)

Approvers:
  - Karthikeyan (https://github.com/karthikeyann)
  - David Wendt (https://github.com/davidwendt)

URL: #8283

cpp/src/copying/contiguous_split.cu

@@ -248,7 +248,6 @@ __device__ void copy_buffer(uint8_t* __restrict__ dst,
  * the actual copy.
  *
  * @param num_src_bufs Total number of source buffers (N)
- * @param num_partitions Number of partitions the each source buffer is split into (M)
  * @param src_bufs Input source buffers (N)
  * @param dst_bufs Desination buffers (N*M)
  * @param buf_info Information on the range of values to be copied for each destination buffer.