fix: to_cloudformation functions of PushEventSource class docstrings

[53ningen]

Issue #, if available:
#1017

I'm sorry if I was wrong but I understand CloudWatch Events trigger is no pull event.
So I think docstrings of to_cloudformation of CloudWatchEvent class and Schedule class have to fix.

• [-] Write/update tests
• [-] make pr passes
• Update documentation
• [-] Verify transformed template deploys and application functions as expected
• [-] Add/update example to examples/2016-10-31

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (develop@275e461). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             develop    #1018   +/-   ##
==========================================
  Coverage           ?   94.81%           
==========================================
  Files              ?       69           
  Lines              ?     3240           
  Branches           ?      631           
==========================================
  Hits               ?     3072           
  Misses             ?       87           
  Partials           ?       81

Impacted Files	Coverage Δ
samtranslator/model/eventsources/push.py	88.46% <ø> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 275e461...10d570e. Read the comment docs.

[53ningen]

rebased on current develop