Fix spelling in documents (#6948)

* Update roxygen2 doc.

Co-authored-by: fis <jm.yuan@outlook.com>

src/tree/gpu_hist/evaluate_splits.cu

@@ -125,7 +125,7 @@ struct UpdateNumeric {
                              EvaluateSplitInputs<GradientSumT> const &inputs,
                              DeviceSplitCandidate *best_split) {
     // Use pointer from cut to indicate begin and end of bins for each feature.
-    uint32_t gidx_begin = inputs.feature_segments[fidx];  // begining bin
+    uint32_t gidx_begin = inputs.feature_segments[fidx];  // beginning bin
     int split_gidx = (scan_begin + threadIdx.x) - 1;
     float fvalue;
     if (split_gidx < static_cast<int>(gidx_begin)) {
@@ -152,7 +152,7 @@ __device__ void EvaluateFeature(
     TempStorageT* temp_storage         // temp memory for cub operations
 ) {
   // Use pointer from cut to indicate begin and end of bins for each feature.
-  uint32_t gidx_begin = inputs.feature_segments[fidx];  // begining bin
+  uint32_t gidx_begin = inputs.feature_segments[fidx];  // beginning bin
   uint32_t gidx_end =
       inputs.feature_segments[fidx + 1];  // end bin for i^th feature
   auto feature_hist = inputs.gradient_histogram.subspan(gidx_begin, gidx_end - gidx_begin);

src/tree/gpu_hist/gradient_based_sampler.cuh

@@ -124,7 +124,7 @@ class ExternalMemoryGradientBasedSampling : public SamplingStrategy {
  * Processing Systems (pp. 3146-3154).
  * \see Zhu, R. (2016). Gradient-based sampling: An adaptive importance sampling for least-squares.
  * In Advances in Neural Information Processing Systems (pp. 406-414).
- * \see Ohlsson, E. (1998). Sequential poisson sampling. Journal of official Statistics, 14(2), 149.
+ * \see Ohlsson, E. (1998). Sequential Poisson sampling. Journal of official Statistics, 14(2), 149.
  */
 class GradientBasedSampler {
  public:

src/tree/gpu_hist/histogram.cu

@@ -17,7 +17,7 @@
 
 namespace xgboost {
 namespace tree {
-// Following 2 functions are slightly modifed version of fbcuda.
+// Following 2 functions are slightly modified version of fbcuda.
 
 /* \brief Constructs a rounding factor used to truncate elements in a sum such that the
    sum of the truncated elements is the same no matter what the order of the sum is.