Fixed 'Bootstwatch' typo in customise.Rmd (#2767)

vignettes/customise.Rmd

@@ -243,7 +243,7 @@ navbar:
   bg: primary
 ```
 
-You generally don't need to set `bg` if you use a bootswatch theme, as pkgdown will pick the `bg` used on the [Bootstwatch preview](https://bootswatch.com/).
+You generally don't need to set `bg` if you use a bootswatch theme, as pkgdown will pick the `bg` used on the [Bootswatch preview](https://bootswatch.com/).
 Similarly, you don't usually need to set `type` because bootstrap will guess it for you.
 If the guess is wrong, you can override with `type: light` or `type: dark` depending on whether the background colour is light (so you need dark text) or `type: dark` if the background is dark (so you need light text).
 Unfortunately, these are defined relative to the page background, so if you have a dark site you'll need to flip `light` and `dark` (a little experimentation should quickly determine what looks best).