Clarify syntax for linguist-generated attribute (#41130)

Co-authored-by: Sharra-writes <[email protected]>

Author: HaraldNordgren

content/repositories/working-with-files/managing-files/customizing-how-changed-files-appear-on-github.md

@@ -20,8 +20,13 @@ Use a _.gitattributes_ file to mark files that match a given "pattern" with the
 
    For example, to mark `search/index.json` as a generated file, add this line to _.gitattributes_:
 
-   ```text
-   search/index.json linguist-generated=true
+   ```gitattributes
+   search/index.json linguist-generated
+   ```
+
+   To unmark a file that would generally be considered _generated_, add this line to _.gitattributes_:
+   ```gitattributes
+   bootstrap.min.css -linguist-generated
    ```
 
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