docs: explicitly show implication operator ==> when introducing it in "Getting Started with Dafny: A Guide"

[das-g]

Explicitly show implication operator ==> when introducing it in "Getting Started with Dafny: A Guide"

Description

• Docs-only change. (Thus user-visible, but only in the documentation.)

• I assume https://dafny.org/latest/OnlineTutorial/guide is rendered from the master branch. If it's instead rendered from the latest release and/or additionally rendered for releases, backporting this to those releases and releasing the result in patch releases (probably together with other changes) should probably be considered, but isn't urgent.

How has this been tested?

I didn't test this change, beyond viewing it in VSCodium's Markdown rendering preview (which might assume a different Markdown dialect). However I'm confident that this change should work in all Markdown dialects.

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

[fabiomadge]

The url you're looking for is: https://dafny.org/dafny/OnlineTutorial/guide

[fabiomadge]

Thank you for this contribution!

[das-g]

The url you're looking for is: https://dafny.org/dafny/OnlineTutorial/guide

Are https://dafny.org/dafny/OnlineTutorial/guide and https://dafny.org/latest/OnlineTutorial/guide built from different sources (e.g. one from master and the other one from a released version)? If so, which one from which source?

[fabiomadge]

/latest is the latest release and comes from here: https://github.com/dafny-lang/dafny-lang.github.io/tree/main/latest
/dafny is master and gets generated by this: https://github.com/dafny-lang/dafny/blob/master/.github/workflows/jekyll.yml