strconv: make code formatting more consistent in doc.go

Replaces " \t" code indentation with "\t".

Issues like this are easy to spot with editor that prints
whitespace charecters.

Change-Id: Ia82877e7c99121bf369fa76e46ba52dff84f36bf
Reviewed-on: https://go-review.googlesource.com/101355Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>