Fix below what value is invalid in default_font_size doc

(cherry picked from commit 575a0b4dfbedc5405e1d7ae3d7bc3f3e0e87b5bd)

doc/classes/Theme.xml

@@ -551,7 +551,7 @@
 		</member>
 		<member name="default_font_size" type="int" setter="set_default_font_size" getter="get_default_font_size" default="-1">
 			The default font size of this theme resource. Used as the default value when trying to fetch a font size value that doesn't exist in this theme or is in invalid state. If the default font size is also missing or invalid, the engine fallback value is used (see [member ThemeDB.fallback_font_size]).
-			Values below [code]0[/code] are invalid and can be used to unset the property. Use [method has_default_font_size] to check if this value is valid.
+			Values below [code]1[/code] are invalid and can be used to unset the property. Use [method has_default_font_size] to check if this value is valid.
 		</member>
 	</members>
 	<constants>