Update pipelines.md

[kkoang]

test code:

# test.nu
nu demo.nu o> file.txt
(open file.txt) == "foo\n" # true

nu demo.nu e> file.txt
(open file.txt) == "barbar\n" # true

nu demo.nu o+e> file.txt
(open file.txt) == "foo\nbarbar\n" # true

[fdncred]

Thanks