chore: run mdformat on gnmi-specification.md

[ElodinLaarz]

Run mdformat on gnmi-specification.md.

Also noted that the link in the table of contents for section 2.7 https://github.com/openconfig/reference/blob/master/rpc/gnmi/gnmi-specification.md#27-gnmi-origin--in-path has a typo (--), which I fixed.

This is just ahead of adding a new section to the file. Splitting it out to make that PR more focused.

Copilot summary:

This pull request makes a series of minor editorial improvements to the rpc/gnmi/gnmi-specification.md file. The main focus is on fixing formatting inconsistencies, improving the clarity of references and links, and making small corrections to whitespace and list formatting.

Formatting and Reference Improvements:

• Standardized the formatting of Markdown links and references throughout the document, especially for external documents and section references. [1] [2] [3] [4]
• Fixed minor formatting issues in the table of contents and section headings, such as correcting escaped underscores and removing extra spaces. [1] [2]

Whitespace and List Formatting:

• Improved whitespace and line wrapping for better readability in several sections, such as path descriptions and sample code blocks. [1] [2] [3] [4]
• Cleaned up the revision history section by adding consistent spacing between version entries and clarifying bullet lists. [1] [2] [3]

Minor Corrections:

• Fixed a numbering issue in a list describing the order of path processing in a SetRequest.
• Made minor clarifications and corrections in footnotes and inline notes for clarity and consistency.
• Added a missing newline in a comment regarding protobuf encoding.