style: enhance font of big-O expressions

Author: Sander Maijers

RELEASES.md

@@ -141,7 +141,7 @@ Performance
 -----------
 
 * [The complexity of comparing variables for equivalence during type 
-  unification is reduced from O(n!) to O(n)[1.9tu]. This leads
+  unification is reduced from _O_(_n_!) to _O_(_n_)][1.9tu]. This leads
   to major compilation time improvement in some scenarios.
 * [`ToString` is specialized for `str`, giving it the same performance
   as `to_owned`][1.9ts].