manual: fix exponents on number types.

Author: graydon

doc/rust.md

@@ -2342,12 +2342,12 @@ The machine types are the following:
 
 
 * The unsigned word types `u8`, `u16`, `u32` and `u64`, with values drawn from
-  the integer intervals $[0, 2^8 - 1]$, $[0, 2^16 - 1]$, $[0, 2^32 - 1]$ and
-  $[0, 2^64 - 1]$ respectively.
+  the integer intervals $[0, 2^8 - 1]$, $[0, 2^{16} - 1]$, $[0, 2^{32} - 1]$ and
+  $[0, 2^{64} - 1]$ respectively.
 
 * The signed two's complement word types `i8`, `i16`, `i32` and `i64`, with
   values drawn from the integer intervals $[-(2^7), 2^7 - 1]$,
-  $[-(2^15), 2^15 - 1]$, $[-(2^31), 2^31 - 1]$, $[-(2^63), 2^63 - 1]$
+  $[-(2^{15}), 2^{15} - 1]$, $[-(2^{31}), 2^{31} - 1]$, $[-(2^{63}), 2^{63} - 1]$
   respectively.
 
 * The IEEE 754-2008 `binary32` and `binary64` floating-point types: `f32` and