Docs fix spelling issues (#13075)

Author: nnsW3

STYLE-GUIDE.md

@@ -22,7 +22,7 @@ Words are important. Pulumi strives to use language that is clear, harmonious, a
 * Ensure that readers are able to scan the headings of a page and get an effective overview of the page's content.
 * Every page should have exactly one `h1`.
 * Headings levels should only increment one level at a time.  E.g., if your previous heading level was an `h2`, the next heading must be an `h2` or an `h3`, but not, e.g., an `h4` or `h5`.
-* Docs and registry headings should use sentence case (i.e., first letter of the first word of is capitalized).
+* Docs and registry headings should use sentence case (i.e., first letter of the first word is capitalized).
 
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