Bump clang-format to 10

doc/sources/contribute.rst

@@ -26,16 +26,16 @@ Coding style
 We use clang-format to format source code consistently.  The
 clang-format configuration file .clang-format is located at the root
 directory.  Since clang-format produces slightly different results
-between versions, we currently use clang-format-9.
+between versions, we currently use clang-format-10.
 
 To detect any violation to the coding style, we recommend to setup git
 pre-commit hook to check coding style of the changes you introduced.
 The pre-commit file is located at the root directory.  Copy it under
 .git/hooks and make sure that it is executable.  The pre-commit script
 uses clang-format-diff.py to detect any style errors.  If it is not in
-your PATH or it exists under different name (e.g., clang-format-diff-9
-in debian), either add it to PATH variable or add git option
-``clangformatdiff.binary`` to point to the script.
+your PATH or it exists under different name (e.g.,
+clang-format-diff-10 in debian), either add it to PATH variable or add
+git option ``clangformatdiff.binary`` to point to the script.
 
 For emacs users, integrating clang-format to emacs is very easy.
 clang-format.el should come with clang distribution.  If it is not