Fix docs for FFT TSNE (#6967)

Wrong description of approximation algorithm for TSNE

Authors:
  - Jinsol Park (https://github.com/jinsolp)
  - Tim Head (https://github.com/betatim)

Approvers:
  - Tim Head (https://github.com/betatim)
  - Victor Lafargue (https://github.com/viclafargue)

URL: #6967

Author: jinsolp

cpp/src/tsne/fft_tsne.cuh

@@ -152,7 +152,8 @@ std::pair<value_t, value_t> min_max(const value_t* Y, const value_idx n, cudaStr
 }
 
 /**
- * @brief Fast Dimensionality reduction via TSNE using the Barnes Hut O(NlogN) approximation.
+ * @brief Fast Dimensionality reduction via TSNE using the fast Fourier transform interpolation
+ * approximation.
  * @param[in] VAL: The values in the attractive forces COO matrix.
  * @param[in] COL: The column indices in the attractive forces COO matrix.
  * @param[in] ROW: The row indices in the attractive forces COO matrix.