Hardcode example output in the README (#19)

Otherwise it doesn't render properly in GitHub, resulting in a very
confusing first impression.

Author: llucax

README.md

@@ -47,21 +47,14 @@ This is line 17
 
 This will show the following block of code in the rendered output:
 
-```text show_lines=":2,4,7,10:12,15:"
+```text
 This is line 1
 This is line 2
-This is line 3
 This is line 4
-This is line 5
-This is line 6
 This is line 7
-This is line 8
-This is line 9
 This is line 10
 This is line 11
 This is line 12
-This is line 13
-This is line 14
 This is line 15
 This is line 16
 This is line 17