Add missing font: component to example. (#2614)

Co-authored-by: Samuel Sieb <samuel-github@sieb.net>
Co-authored-by: Jesse Hills <3060199+jesserockz@users.noreply.github.com>
fixes https://github.com/esphome/issues/issues/4065

components/display/st7789v.rst

@@ -48,6 +48,15 @@ to an ESP module.
         lambda: |-
           it.print(0, 0, id(font), "Hello World!");
 
+    font:
+      - file: "gfonts://Roboto"
+        id: font
+        size: 20
+
+.. note::
+
+    For more information about the font options see: :ref:`display-fonts`.
+
 .. warning::
 
     When using the TTGO T-Display module, the GPIO pin numbers above *cannot be changed* as they are