fmt: fix scientific notation in docs

Fixes #12340.

Change-Id: I17a8b3711a8593ec60882a0dcadb38f0cc138f4b
Reviewed-on: https://go-review.googlesource.com/13949Reviewed-by: Rob Pike <r@golang.org>

src/fmt/doc.go

@@ -34,8 +34,8 @@
 		%b	decimalless scientific notation with exponent a power of two,
 			in the manner of strconv.FormatFloat with the 'b' format,
 			e.g. -123456p-78
-		%e	scientific notation, e.g. -1234.456e+78
-		%E	scientific notation, e.g. -1234.456E+78
+		%e	scientific notation, e.g. -1.234456e+78
+		%E	scientific notation, e.g. -1.234456E+78
 		%f	decimal point but no exponent, e.g. 123.456
 		%F	synonym for %f
 		%g	%e for large exponents, %f otherwise