docs: add missing # (TheAlgorithms#50)

* docs: add missing `#`

* docs: update DIRECTORY.md

* docs: remove ` of a Circle`

Co-authored-by: Zoom <ZoomRmc@users.noreply.github.com>

* docs: update DIRECTORY.md

---------

Co-authored-by: Zoom <ZoomRmc@users.noreply.github.com>

Author: 2 people

dynamic_programming/catalan_numbers.nim

@@ -1,4 +1,4 @@
-# Catalan Numbers
+## Catalan Numbers
 #[
     The Catalan numbers are a sequence of natural numbers that occur in the
     most large set of combinatorial problems.

dynamic_programming/viterbi.nim

@@ -1,4 +1,4 @@
-# Viterbi
+## Viterbi
 {.push raises: [].}
 
 type

maths/arc_length.nim

@@ -1,4 +1,4 @@
-# Arc Length of a Circle
+## Arc Length
 ## https://en.wikipedia.org/wiki/Arc_length
 import std/math
 {.push raises: [].}