[NFC] Change whitespace to force docs rebuild

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@289205 91177308-0d34-0410-b5e6-96231b3b80d8

docs/UsingLibcxx.rst

@@ -182,3 +182,4 @@ thread safety annotations.
       using Tup = std::tuple<std::string, int, std::error_code>;
       return Tup{"hello world", 42}; // explicit constructor called. OK.
     }
+