stats.lua: set border size to have similar relative thickness as OSD

It is 6% of font size. Little bit more relativelly to OSD, but stats has
smaller text, so we need to compensate for that.

DOCS/man/stats.rst

@@ -165,7 +165,7 @@ Configurable Options
     Color of the text.
 
 ``border_size``
-    Default: 2
+    Default: 1.2
 
     Size of border drawn around the font.
 

player/lua/stats.lua

@@ -56,7 +56,7 @@ local o = {
     font_mono = "monospace",   -- monospaced digits are sufficient
     font_size = 20,
     font_color = "",
-    border_size = 2,
+    border_size = 1.2,
     border_color = "",
     shadow_x_offset = 0.0,
     shadow_y_offset = 0.0,