Fix formatting of code in formatter.md (#70)

Author: kdheepak

docs/src/userguide/formatter.md

@@ -21,7 +21,7 @@ f(x) = 2x + 3
 print(f'(2))
 
 open("myfile.txt", "w") do io
-	write(io, "Hello world!")
+    write(io, "Hello world!")
 end;
 ```