Fix typo

Fix #328

Author: dawei-wang

content/computability/recursive-functions/pr-functions.tex

@@ -124,7 +124,7 @@
 
 \begin{prob}
   Prove \olref[cmp][rec][prf]{prop:mult-pr} by showing that the
-  primitive recursive definition of $\Mult$ is can be put into the
+  primitive recursive definition of $\Mult$ can be put into the
   form required by \olref[cmp][rec][prf]{defn:primitive-recursion} and
   showing that the corresponding functions $f$ and~$g$ are primitive
   recursive.