Update API Reference

API-Reference.md

@@ -12,6 +12,7 @@ The following functions are provided by the library:
 |s.circ(x, y, r, color=0)|Circle|
 |s.oval(x, y, rx, ry, color = 0)|Oval|
 |s.line(x1, y1, x2, y2, color=0)|Line segment|
+|s.linewidth(thickness)|Change line thickness|
 |s.text(x, y, message, color=3, size="normal")|Text rendering|
 
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