[AIRFLOW-XXX] Correct typos in UPDATING.md master

[dlamblin]

Make sure you have checked all steps below.

Jira

• My PR addresses fixing a typo in the documentation and so prepends commit with [AIRFLOW-XXX].

Description

Applies to master; pull request 4241 applies to `v1-10-stable.

• Started with "habe", "serever" and "certificiate" needing to be:
  "have", "server", and "certificate".
• Ran a check, ignoring British and US accepted spellings.
• Kept jargon. EG admin, aync, auth, backend, config, dag, s3, utils, etc.
• Took exception to:
  • "num of dag run" meaning "number of dag runs",
  • "upness" is normally for quarks,
  • "url" being lower-case, and
  • sftp example having an excess file ending.
• Python documentation writes "builtin" hyphenated, cases "PYTHONPATH".
• Gave up on mixed use of "dag" and "DAG" as well as long line lengths.

Tests

• My PR does not need testing for this extremely good reason: It is scoped to documentation

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

• Updates only existing documentation.

Code Quality

• Passes flake8

[dlamblin]

Not sure how you mange the diffs from the master and stable branch to this doc.
See also #4241