Update the example of BS_Stroustrup to match what is done by clang-format

Summary:
reported here https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=911561

clang-format-7 -style="{BreakBeforeBraces: Stroustrup}" wasn't doing
the same as the doc

Reviewers: krasimir

Reviewed By: krasimir

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D53520

llvm-svn: 345371