[clang-format] vim integration: Mention python3 variant of bindings

[jsilvanus]

Summary

The instructions in the documentation only mentioned how to include
bindings for clang-format into vim using python2. Add the instructions
for python3 which were already present in the source comments.

NOTE: Clang-Format Team Automated Review Comment

It looks like your clang-format review does not contain any unit tests, please try to ensure all code changes have a unit test (unless this is an NFC or refactoring, adding documentation etc..)

Add your unit tests in clang/unittests/Format and you can build with ninja FormatTests. We recommend using the verifyFormat(xxx) format of unit tests rather than EXPECT_EQ as this will ensure you change is tolerant to random whitespace changes (see FormatTest.cpp as an example)

For situations where your change is altering the TokenAnnotator.cpp which can happen if you are trying to improve the annotation phase to ensure we are correctly identifying the type of a token, please add a token annotator test in TokenAnnotatorTest.cpp

[jsilvanus]

Don't change proposed key binding from K to I,
just add the python3 part.

[MyDeveloperDay]

LGTM

clang/docs/ClangFormat.rst

 :program:`clang-format` standalone tool on your current buffer, optionally
 selecting regions to reformat. The integration has the form of a `python`-file
 which can be found under `clang/tools/clang-format/clang-format.py`.
 
 This can be integrated by adding the following to your `.vimrc`:
 
 .. code-block:: vim
 
+  if has('python')
     map <C-K> :pyf <path-to-this-file>/clang-format.py<cr>
     imap <C-K> <c-o>:pyf <path-to-this-file>/clang-format.py<cr>
+  elseif has('python3')
+    map <C-K> :py3f <path-to-this-file>/clang-format.py<cr>
+    imap <C-K> <c-o>:py3f <path-to-this-file>/clang-format.py<cr>
+  endif
 
 The first line enables :program:`clang-format` for NORMAL and VISUAL mode, the
 second line adds support for INSERT mode. Change "C-K" to another binding if
 you need :program:`clang-format` on a different key (C-K stands for Ctrl+k).
 
 With this integration you can press the bound key and clang-format will
 format the current line in NORMAL and INSERT mode or the selected region in
 VISUAL mode. The line or region is extended to the next bigger syntactic