docs: fix docs for the length field

[Dr-Emann]

Using the Broadcom repo as canonical:

The field corresponds to ulTotalCompressedLength, and it's set to the sum of the payload size:

https://github.com/Broadcom/aeolus/blob/bdc6835b7e852d446cde086c1a902f89c1dff588/ProgramStore/ProgramStore.cpp#L493-L494

The same also appears to be true in bcm2-utils:

bcm2-utils/psextract.cc

Lines 76 to 86 in 5909b3b

	void do_extract(istream& in, const ps_header& ps, size_t length = 0)
	{
	if (!length) {
	length = ps.length();
	}
	auto buf = make_unique<char[]>(length);
	if (!in.read(buf.get(), length)) {
	throw runtime_error("read error (data)");
	}

, where it reads 'length' bytes after it's already read the header.

See binwalk-ng/binwalk-ng#32 (comment)