[AIRFLOW-XXX] Fix incorrect units in docs for metrics using Timers

[kaxil]

Make sure you have checked all steps below.

Jira

• My PR addresses the following Airflow Jira issues and references them in the PR title. For example, "[AIRFLOW-XXX] My Airflow PR"
  • https://issues.apache.org/jira/browse/AIRFLOW-XXX

Description

• Here are some details about my PR, including screenshots of any UI changes:

https://statsd.readthedocs.io/en/v2.1.2/timing.html - Stats timers would emit time in milliseconds.

Tests

• My PR adds the following unit tests OR does not need testing for this extremely good reason:
  n/a - doc change

Commits

• My commits all reference Jira issues in their subject lines, and I have squashed multiple commits if they address the same issue. In addition, my commits follow the guidelines from "How to write a good git commit message":
  1. Subject is separated from body by a blank line
  2. Subject is limited to 50 characters (not including Jira issue reference)
  3. Subject does not end with a period
  4. Subject uses the imperative mood ("add", not "adding")
  5. Body wraps at 72 characters
  6. Body explains "what" and "why", not "how"

Documentation

• In case of new functionality, my PR adds documentation that describes how to use it.
  • All the public functions and the classes in the PR contain docstrings that explain what it does
  • If you implement backwards incompatible changes, please leave a note in the Updating.md so we can assign it to a appropriate release

[codecov-io]

Codecov Report

Merging #6152 into master will increase coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #6152      +/-   ##
==========================================
+ Coverage   80.05%   80.05%   +<.01%     
==========================================
  Files         607      607              
  Lines       35044    35044              
==========================================
+ Hits        28054    28055       +1     
+ Misses       6990     6989       -1

Impacted Files	Coverage Δ
airflow/utils/dag_processing.py	58.98% <0%> (+0.18%)	⬆️

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