Fix typo in docs/designers-developers/key-concepts.md (#13005)

docs/designers-developers/key-concepts.md

@@ -117,7 +117,7 @@ We chose instead to try and find a way to keep the formality and explicitness an
 
 Of these options a novel approach was suggested that by storing data in HTML comments we would know that we wouldn't break the rest of the HTML in the document, that browsers should ignore it, and that we could simplify our approach to parsing the document.
 
-Unique to comments is that they cannot legitimately exist in ambiguous places, such as inside of HTML attributes like `<img alt='data-id="14"'>`. Comments are also quite permissive. Whereas HTML attributes are complicated to parse properly, comments are quite easily described by a leading `<!--` followed by anything except `--` until the first `-->`. This simplicity and permisiveness means that the parser can be implemented in several ways without needing to understand HTML properly and we have the liberty to use more convenient syntax inside of the comment—we only need to escape double-hyphen sequences. We take advantage of this in how we store block attributes: JSON literals inside the comment.
+Unique to comments is that they cannot legitimately exist in ambiguous places, such as inside of HTML attributes like `<img alt='data-id="14"'>`. Comments are also quite permissive. Whereas HTML attributes are complicated to parse properly, comments are quite easily described by a leading `<!--` followed by anything except `--` until the first `-->`. This simplicity and permissiveness means that the parser can be implemented in several ways without needing to understand HTML properly and we have the liberty to use more convenient syntax inside of the comment—we only need to escape double-hyphen sequences. We take advantage of this in how we store block attributes: JSON literals inside the comment.
 
 After running this through the parser we're left with a simple object we can manipulate idiomatically and we don't have to worry about escaping or unescaping the data. It's handled for us through the serialization process. Because the comments are so different from other HTML tags and because we can perform a first-pass to extract the top-level blocks, we don't actually depend on having fully valid HTML!