dbt compile help text is missing a comma

[sethwoodworth]

Describe the bug

dbt -h outputs:

...
    compile             Generates executable SQL from source model, test, and
                        analysis files. Compiled SQL files are written to the
                        target/ directory.
...

but a 'source model' is not a construct within dbt, there are sources and models.

Steps To Reproduce

dbt -h

Expected behavior

I'd expect a comma between each object the help text describes, especially since the oxford comma is used before the word 'and'

System information

Which database are you using dbt with?

• postgres
• redshift
• bigquery
• snowflake
• other (specify: ____________)

The output of dbt --version:

installed version: 0.15.1

(but this is still true in dbt 0.16.x and in the current master branch)
The operating system you're using:
debian buster
The output of python --version:
Python 3.7.4.