docs: fix hash1 field in firmware header

docs/hardware/model-t/boot.md

@@ -161,7 +161,7 @@ Total length of firmware header is always 1024 bytes.
 | 0x0016 | 1      | fix_vpatch | version of last critical bugfix (patch) |
 | 0x0017 | 1      | fix_vbuild | version of last critical bugfix (build) |
 | 0x0018 | 8      | reserved | not used yet (zeroed) |
-| 0x0020 | 32     | hash1 | hash of the first code chunk (128 - 1 KiB), this excludes the header |
+| 0x0020 | 32     | hash1 | hash of the first code chunk excluding both the firmware and the vendor header (128 - 1 - \[vendor header length\] KiB) |
 | 0x0040 | 32     | hash2 | hash of the second code chunk (128 KiB), zeroed if unused |
 | ...    | ...    | ... | ... |
 | 0x0200 | 32     | hash16 | hash of the last possible code chunk (128 KiB), zeroed if unused |