rustdoc: Fix file locking on windows

If the dwShareMode parameter is 0 on windows, it "prevents other processes from
opening a file or device if they request delete, read, or write access", which
is the opposite of what we want! This changes the 0 parameter to something which
will allow multiple processes to open the file and then lock it.
auto merge of #13043 : alexcrichton/rust/fix-rustdoc-windows, r=brson

If the dwShareMode parameter is 0 on windows, it "prevents other processes from
opening a file or device if they request delete, read, or write access", which
is the opposite of what we want! This changes the 0 parameter to something which
will allow multiple processes to open the file and then lock it.