Fix spelling in documents (#6948)

* Update roxygen2 doc.

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

doc/python/callbacks.rst

@@ -4,7 +4,7 @@ Callback Functions
 
 This document gives a basic walkthrough of callback function used in XGBoost Python
 package.  In XGBoost 1.3, a new callback interface is designed for Python package, which
-provides the flexiblity of designing various extension for training.  Also, XGBoost has a
+provides the flexibility of designing various extension for training.  Also, XGBoost has a
 number of pre-defined callbacks for supporting early stopping, checkpoints etc.
 
 

doc/python/python_api.rst

@@ -1,6 +1,6 @@
 Python API Reference
 ====================
-This page gives the Python API reference of xgboost, please also refer to Python Package Introduction for more information about python package.
+This page gives the Python API reference of xgboost, please also refer to Python Package Introduction for more information about the Python package.
 
 .. contents::
   :backlinks: none

doc/python/python_intro.rst

@@ -1,7 +1,7 @@
 ###########################
 Python Package Introduction
 ###########################
-This document gives a basic walkthrough of xgboost python package.
+This document gives a basic walkthrough of the xgboost package for Python.
 
 **List of other Helpful Links**
 
@@ -24,7 +24,7 @@ Data Interface
 --------------
 The XGBoost python module is able to load data from:
 
-- LibSVM text format file
+- LIBSVM text format file
 - Comma-separated values (CSV) file
 - NumPy 2D array
 - SciPy 2D sparse array
@@ -36,7 +36,7 @@ The XGBoost python module is able to load data from:
 
 The data is stored in a :py:class:`DMatrix <xgboost.DMatrix>` object.
 
-* To load a libsvm text file or a XGBoost binary file into :py:class:`DMatrix <xgboost.DMatrix>`:
+* To load a LIBSVM text file or a XGBoost binary file into :py:class:`DMatrix <xgboost.DMatrix>`:
 
   .. code-block:: python