spelling correction.

while mat2 has both a thread model (a thread pool that strips metadata
in parallel) and a threat model (a list of malicious adversaries and
their capabilities that we are trying to defeat), i think this
paragraph is talking about the latter.

doc/implementation_notes.md

@@ -64,7 +64,7 @@ to the filesystem. This ensures that every metadata is removed.
 XML attacks
 -----------
 
-Since our thread model conveniently excludes files crafted to specifically
+Since our threat model conveniently excludes files crafted to specifically
 bypass MAT2, fileformats containing harmful XML are out of our scope.
 But since MAT2 is using [etree](https://docs.python.org/3/library/xml.html#xml-vulnerabilities)
 to process XML, it's "only" vulnerable to DoS, and not memory corruption: