Repair mojibake in expected.hpp

[BillyONeal]

The affected characters herein triggered warnings:

C:\Dev\vcpkg\buildtrees\opentracing\src\b67575dab0-0250653c81.clean\
    3rd_party\include\opentracing/expected/expected.hpp(1):
warning C4828: The file contains a character starting at offset 0x4a77
that is illegal in the current source character set (codepage 65001).