README.md: fix misleading sentence

Problem: a sentence in the README doesn't make sense, it sets up
an "either... or" clause but then doesn't follow up with the "or"
part.

Fix the sentence.

README.md

@@ -28,8 +28,8 @@ If you want to generate the container manually, this is also an option! First bu
 ```bash
 $ docker build -f ./.devcontainer/Dockerfile -t flux-docs .
 ```
-This will build the base environment. You can then bind your container to the present working directory to
-build, either interactively:
+This will build the base environment. You can then bind your container to the present
+working directory to build:
 
 ```bash
 $ docker run -it --rm -v $PWD/:/workspace/flux-docs flux-docs flux start make html