Fix typo in comment for walk function

Author: lrvideckis

library/data_structures_[l,r)/bit_uncommon/walk.hpp

@@ -1,5 +1,5 @@
 //! Requires sum of [i,i] >= 0
-//! Returns min r st sum of [0,r] >= sum
+//! Returns min r s.t. sum of [0,r] >= sum
 //! Returns n if sum of [0,n-1] < sum
 int walk(ll sum) {
   if (sum <= 0) return -1;