Set go version to 1.15

0x2BadB002 · 0x2BadB002 · commit 14b930bcb820 · 2021-12-30T02:42:22.000+03:00
diff --git a/go.mod b/go.mod
@@ -1,6 +1,6 @@
 module github.com/Pavel7004/GraphPlot
 
-go 1.17
+go 1.15
 
 require (
 	github.com/pborman/getopt/v2 v2.1.0
