[w3c/webcomponents] Fix some typos, missing formating and erroneous Markdown in Template-Instantiation.md (#699)

Hi, this PR contains fixes for some typos, missing formatting and erroneous Markdown I encountered while reading the proposal. Please check each change, I am not always sure whether the original version was on purpose.
You can view, comment on, or merge this pull request online at:

  https://github.com/w3c/webcomponents/pull/699

-- Commit Summary --

  * Fix some errors in Template-Instantiation.md

-- File Changes --

    M proposals/Template-Instantiation.md (25)

-- Patch Links --

https://github.com/w3c/webcomponents/pull/699.patch
https://github.com/w3c/webcomponents/pull/699.diff

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