A previous rework of the directory traversal mitigation example in
I3f8d3760daceb9e62396ae21b0d915ae07eff303 was not correctly cleaned
up, and left some unintended startswith method invocations behind.
Get rid of those, and also correct a wrong parameter name in the
main function while we're at it, as well as fixing some incorrect
indentation.
Change-Id: Ie5347f3b6cc8e689440db0aaf552d52ad37c231c
Closes-Bug: #1928544
b05ba12445