fmt: use tabs for indentation

Replace 24 spaces added in CL 389434 with 3 tabs,
so the new line is indented like other lines around it.

Updates #51419.

Change-Id: Ic3e50023a01f233c52dda53c36de2c461222d95c
Reviewed-on: https://go-review.googlesource.com/c/go/+/390674
Trust: Dmitri Shuralyov <dmitshur@golang.org>
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Adam Shannon <adamkshannon@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/fmt/doc.go

@@ -125,7 +125,7 @@
 			put spaces between bytes printing strings or slices in hex (% x, % X)
 		0	pad with leading zeros rather than spaces;
 			for numbers, this moves the padding after the sign;
-                        ignored for strings, byte slices and byte arrays
+			ignored for strings, byte slices and byte arrays
 
 	Flags are ignored by verbs that do not expect them.
 	For example there is no alternate decimal format, so %#d and %d