Windows package: remove spurious antislash

Introduced in 2ae88722
11 jobs for master in 68 minutes and 52 seconds (queued for 4 seconds)
Status Job ID Name Coverage
  Build
passed #15945
build_binaries

00:12:06

passed #15946
build_doc

00:02:29

 
  Test And Pkg
passed #15949
macOS
pkg_macOS

00:13:34

passed #15947
pkg_source

00:01:49

passed #15948
pkg_windows

00:28:03

passed #15953
test_dynare++

00:11:40

passed #15950
test_matlab

00:54:59

manual #15952
allowed to fail manual
test_octave
manual #15951
allowed to fail manual
test_old_matlab
 
  Deploy
passed #15954
restricted
deploy_manual_unstable

00:00:26

passed #15955
restricted
deploy_snapshot_unstable

00:01:19