scripts: gen_relocate_app: Fix a typo in parse_input_string()

This is a follow-up to commit b1a3ce40.

`parse_input_string()` needs to use `:NOCOPY:`, not `:NOCOPY`, when
partitioning input lines, otherwise, when a line contains the NOCOPY
flag, the file name returned by the function starts with `:` and the
file cannot be then found by the script.
Such problem can be observed in the code_relocation_nocopy sample,
which without this fix does not actually relocate code from ext_code.c.

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>