use proper footnote syntax for references

The previous syntax was causing rustdoc to interpret them as links.

src/libcore/num/flt2dec/strategy/dragon.rs

@@ -8,12 +8,11 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-/*!
-Almost direct (but slightly optimized) Rust translation of Figure 3 of [1].
-
-[1] Burger, R. G. and Dybvig, R. K. 1996. Printing floating-point numbers
-    quickly and accurately. SIGPLAN Not. 31, 5 (May. 1996), 108-116.
-*/
+//! Almost direct (but slightly optimized) Rust translation of Figure 3 of "Printing
+//! Floating-Point Numbers Quickly and Accurately"[^1].
+//!
+//! [^1]: Burger, R. G. and Dybvig, R. K. 1996. Printing floating-point numbers
+//!   quickly and accurately. SIGPLAN Not. 31, 5 (May. 1996), 108-116.
 
 use cmp::Ordering;
 

src/libcore/num/flt2dec/strategy/grisu.rs

@@ -8,13 +8,12 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-/*!
-Rust adaptation of Grisu3 algorithm described in [1]. It uses about
-1KB of precomputed table, and in turn, it's very quick for most inputs.
-
-[1] Florian Loitsch. 2010. Printing floating-point numbers quickly and
-    accurately with integers. SIGPLAN Not. 45, 6 (June 2010), 233-243.
-*/
+//! Rust adaptation of the Grisu3 algorithm described in "Printing Floating-Point Numbers Quickly
+//! and Accurately with Integers"[^1]. It uses about 1KB of precomputed table, and in turn, it's
+//! very quick for most inputs.
+//!
+//! [^1]: Florian Loitsch. 2010. Printing floating-point numbers quickly and
+//!   accurately with integers. SIGPLAN Not. 45, 6 (June 2010), 233-243.
 
 use num::diy_float::Fp;
 use num::flt2dec::{Decoded, MAX_SIG_DIGITS, round_up};