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

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.

Differential Revision: https://reviews.llvm.org/D153338

Change-Id: I25fdbd36f0c7e745061908be8e26f68cb31c7dd5