docs: correct publish input default to false in README

[JamBalaya56562]

The README's input table claims publish defaults to true, but the
actual default in both this action and the underlying AWS Lambda API
is false:

• action.yml has no default: for the publish input, so an unset
  input is undefined.
• validations.js initializes publish to false and falls back to
  false on any parse error.
• index.js only adds Publish to CreateFunction / UpdateFunctionCode
  when publish !== undefined, so omitting the input means AWS uses
  its own default.
• The AWS Lambda CreateFunction API treats Publish as an optional
  Boolean that is not required; the documented behavior is that you
  must set it to true to publish, and omitting it does not publish.
  https://docs.aws.amazon.com/lambda/latest/api/API_CreateFunction.html

Issue #58 reports this discrepancy and offers two options: change the
docs, or change the default behavior. Changing the default to true
(as attempted in PR #63) would be a breaking change for every
existing user — every deploy would suddenly publish a new version,
and the action's behavior would diverge from the AWS API default.
The maintainer pushback on #63 points to this as well.

This change takes the docs-only path: align the documentation with
the actual (and AWS-standard) behavior. No code change.

Refs #58, #63.