[COFF] - Fixed format in writeOutSecLine()

The same as https://reviews.llvm.org/rL292102,
fixes next testcases under msvs2015/win32:

25>  Failing Tests (3):
25>      lld :: COFF/lldmap.test
25>      lld :: COFF/weak-external.test
25>      lld :: COFF/weak-external3.test

llvm-svn: 292104