Use squiggly heredocs in Gem::Specification#description documentation, so it doesn't add leading whitespace

[bravehager]

What was the end-user or developer problem that led to this PR?

Using a non-squiggly heredoc adds leading whitespace in the gem description. rubygems.org seems to handle this fine (by relying on HTML ignoring it), but in other contexts it can make it difficult to render the text (e.g. in an editor using an LSP).

(also see rubygems/guides#352)

What is your fix for the problem, implemented in this PR?

Use a squiggly heredoc in the specification documentation.

Make sure the following tasks are checked

• Describe the problem / feature
• Write tests for features and bug fixes
• Write code to solve the problem
• Make sure you follow the current code style and write meaningful commit messages without tags

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