Shading: Fix type of texscreen builtin

Fixes godotengine/godot#3651.

learning/features/shading/shading_language.rst

@@ -207,7 +207,7 @@ follows:
 +-----------------------------------------------------------------------+---------------------------------------------+
 | color *texcube* ( texture, vec3 )                                     | Read from a cubemap                         |
 +-----------------------------------------------------------------------+---------------------------------------------+
-| color *texscreen* ( vec2 )                                            | Read from screen (generates a copy)         |
+| vec3 *texscreen* ( vec2 )                                             | Read RGB from screen (generates a copy)     |
 +-----------------------------------------------------------------------+---------------------------------------------+
 
 Built-in variables