Fix spelling in documents (#6948)

* Update roxygen2 doc.

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

src/data/adapter.h

@@ -42,7 +42,7 @@ namespace data {
  * This abstraction allows us to read through different sparse matrix formats
  * using the same interface. In particular we can write a DMatrix constructor
  * that uses the same code to construct itself from a CSR matrix, CSC matrix,
- * dense matrix, csv, libsvm file, or potentially other formats. To see why this
+ * dense matrix, CSV, LIBSVM file, or potentially other formats. To see why this
  * is necessary, imagine we have 5 external matrix formats and 5 internal
  * DMatrix types where each DMatrix needs a custom constructor for each possible
  * input. The number of constructors is 5*5=25. Using an abstraction over the
@@ -736,7 +736,7 @@ class IteratorAdapter : public dmlc::DataIter<FileAdapterBatch> {
 
   size_t columns_;
   size_t row_offset_;
-  // at the beinning.
+  // at the beginning.
   bool at_first_;
   // handle to the iterator,
   DataIterHandle data_handle_;