Representing Ed25519 p curve in hexadecimal byte string
The constant is named minusp, thus it's $-p \pmod {2^{256}}$, or $2^{255} + 19$, encoded in little-endian.

Assuming you want the hex string in big-endian, which is the way numbers are usually written, you get:

$p = 2^{255}-19$ = 7FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFED

$2^{256} - p = 2^{255}+19$ = 8000000000000000000000000000000000000000000000000000000000000013