Fix typo.

[rolfschr]

Dear Airflow Maintainers,

Please accept this PR that fixes a typo.

Let me know if I need to create an JIRA issue for that. I could not find "typo" as Types of Contribution in the guidelines for contributing.

[codecov-io]

Codecov Report

Merging #2059 into master will not change coverage.

@@          Coverage Diff           @@
##           master   #2059   +/-   ##
======================================
  Coverage    67.6%   67.6%           
======================================
  Files         139     139           
  Lines       10578   10578           
======================================
  Hits         7151    7151           
  Misses       3427    3427

Impacted Files	Coverage Δ
airflow/models.py	86.16% <ø> (ø)	✅

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[r39132]

Thx for the contribution.. I'm sure you may find other such documentation fixes for your first commit. Please get familiar with our Contributor guidelines (e.g. open a jira, preface commit title with jira issue id) and add a few more doc updates (typo fixes) here. For example, I recall seeing quite a few typos and grammatical issues in UPDATING.md. You may want to fix those if they persist.

https://cwiki.apache.org/confluence/display/AIRFLOW/Contributors%27+Guide

[rolfschr]

Hi,
you can close this PR. I will not open a jira issue for this nor search for other typos. Too much overhead for everbody.

[jlowin]

@rolfschr only you can close your own PR.