Documentation: Fix an exponent

Signed-off-by: Horst H. von Brand <vonbrand@inf.utfsm.cl>

Author: vonbrand

doc/bn.tex

@@ -2272,7 +2272,7 @@ \section{Primality Testing}
 
 If  $t$ is set to a negative value the test will run the deterministic Miller--Rabin test for the
 primes up to $3\,317\,044\,064\,679\,887\ 385\,961\,981$\footnote{The semiprime $1287836182261\cdot
-    2575672364521$ with both factors smaller than $2^64$. An alternative with all factors smaller
+    2575672364521$ with both factors smaller than $2^{64}$. An alternative with all factors smaller
   than
   $2^32$ is $4290067842\cdot 262853\cdot 1206721\cdot 2134439 + 3$}. That limit has to be checked
 by