fix a quoting issue in filename attribute

Author: cderv

docs/extensions/brand.qmd

@@ -80,7 +80,7 @@ Note that this example, and brand extensions in general, require a `_quarto.yml`
 
 We've provided the most basic Quarto project for this example:
 
-``` {.yaml filename="_quarto.yml`}
+``` {.yaml filename="_quarto.yml"}
 project:
   type: default
 ```