math/big: fix alignment in Float.Parse docs

math/big: fix alignment in Float.Parse docs

Leading spaces in a couple of lines instead of tabs cause those to be
misaligned (as seen on <https://golang.org/pkg/math/big/#Float.Parse>):

<<<
	number   = [ sign ] [ prefix ] mantissa [ exponent ] | infinity .
	sign     = "+" | "-" .
     prefix   = "0" ( "x" | "X" | "b" | "B" ) .
	mantissa = digits | digits "." [ digits ] | "." digits .
	exponent = ( "E" | "e" | "p" ) [ sign ] digits .
	digits   = digit { digit } .
	digit    = "0" ... "9" | "a" ... "z" | "A" ... "Z" .
     infinity = [ sign ] ( "inf" | "Inf" ) .
>>>

Replace the leading spaces with tabs so that those align well.

Change-Id: Ibba6cd53f340001bbd929067dc587feb071dc3bd
Reviewed-on: https://go-review.googlesource.com/31830
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>