[AIRFLOW-2083] Docs: Use "its" instead of "it's" where appropriate

[wrp]

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-2083

Description

• Here are some details about my PR, including screenshots of any UI changes:
  Fix trivial spelling/grammar errors

Tests

• My PR adds the following unit tests OR does not need testing for this extremely good reason: trivial changes to documentation. No testing appropriate

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
  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"

• Passes git diff upstream/master -u -- "*.py" | flake8 --diff

[jgao54]

Flake8 nitty stuff

[codecov-io]

Codecov Report

Merging #3020 into master will decrease coverage by 0.08%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3020      +/-   ##
==========================================
- Coverage   73.17%   73.09%   -0.09%     
==========================================
  Files         177      177              
  Lines       12412    12412              
==========================================
- Hits         9083     9073      -10     
- Misses       3329     3339      +10

Impacted Files	Coverage Δ
airflow/jobs.py	79.57% <ø> (-0.45%)	⬇️
airflow/bin/cli.py	54.8% <ø> (ø)	⬆️
airflow/utils/dates.py	84.21% <ø> (ø)	⬆️
airflow/ti_deps/deps/trigger_rule_dep.py	95.16% <ø> (ø)	⬆️
airflow/www/views.py	71.68% <ø> (ø)	⬆️
airflow/task/task_runner/bash_task_runner.py	93.33% <0%> (-6.67%)	⬇️
airflow/utils/helpers.py	51.14% <0%> (-2.88%)	⬇️
airflow/models.py	87.07% <0%> (+0.04%)	⬆️

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