LangRef: attempt to fix line breaks

The line breaks around the "m:<mangling>" text in the Data Layout section
look weird. Let's see if this helps.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@199285 91177308-0d34-0410-b5e6-96231b3b80d8
LangRef.rst: fix LangRef data layout text about m specifier, take 2

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@199286 91177308-0d34-0410-b5e6-96231b3b80d8