Getting started exercise fails on Linux

In the Get Started, Part 2 exercise using Python, Flash and Redis, the build step fails. (Googling for this has not so far been encouraging.) It is true that I have neither Flash nor Redis installed on my host (Linux Mint 19, up-to-date), but the exercise leads me to believe that it’s a particular point that I do not need to have them installed. Also, user russ is a member of group docker. I don’t believe permissions are at fault.

Could I get just a push down the road here? Thanks.

russ@nargothrond:~/dev/docker-dev/exercise-1$ docker build -t friendlyhello .
Sending build context to Docker daemon   5.12kB
Step 1/7 : FROM python:2.7-slim
 ---> 14dad3ead5f4
Step 2/7 : WORKDIR /app
 ---> Using cache
 ---> 6623958da619
Step 3/7 : COPY . /app
 ---> Using cache
 ---> 26bada283754
Step 4/7 : RUN pip install --trusted-host -r requirements.txt
 ---> Running in 9ca54387b566
Collecting Flash (from -r requirements.txt (line 1))
  Could not find a version that satisfies the requirement Flash (from -r requirements.txt (line 1)) (from versions: )
No matching distribution found for Flash (from -r requirements.txt (line 1))
The command '/bin/sh -c pip install --trusted-host -r requirements.txt' returned a non-zero code: 1

Oops. I had brainlessly typed “Flash” instead of “Flask” in requirements.txt.