Update is-anagram.md

Author: neetcode-gh

articles/is-anagram.md

@@ -271,9 +271,9 @@ class Solution {
 ### Time & Space Complexity
 
 * Time complexity: $O(n + m)$
-* Space complexity: $O(1)$
+* Space complexity: $O(1)$ since we have at most 26 different characters.
 
-> Where $n$ is the length of string $s$ and $m$ is the length of string $t$.
+> Where $n$ is the length of string $s$ and $m$ is the length of string $t$. 
 
 ---
 
@@ -439,6 +439,6 @@ class Solution {
 ### Time & Space Complexity
 
 * Time complexity: $O(n + m)$
-* Space complexity: $O(1)$
+* Space complexity: $O(1)$ since we have at most 26 different characters.
 
-> Where $n$ is the length of string $s$ and $m$ is the length of string $t$.
+> Where $n$ is the length of string $s$ and $m$ is the length of string $t$.