doc: switch from spaces to tabs for indentation in example code

This is the standard that gofmt uses.

Change-Id: I30e70611ccd0d3bbaa500fa84e9d1ba75970f0bc
GitHub-Last-Rev: b2b7af0df9759ad447b6fd3551ea356804d136b2
GitHub-Pull-Request: golang/go#24802
Reviewed-on: https://go-review.googlesource.com/106123Reviewed-by: Ian Lance Taylor <iant@golang.org>

doc/install-source.html

@@ -307,7 +307,7 @@ package main
 import "fmt"
 
 func main() {
-    fmt.Printf("hello, world\n")
+	fmt.Printf("hello, world\n")
 }
 </pre>
 

doc/install.html

@@ -242,7 +242,7 @@ package main
 import "fmt"
 
 func main() {
-    fmt.Printf("hello, world\n")
+	fmt.Printf("hello, world\n")
 }
 </pre>