fixed the using namespace std

Author: dibyam-jalan27

snippets/cpp/math-and-numbers/sieve-of-eratosthenes.md

@@ -7,12 +7,9 @@ author: dibyam-jalan27
 
 ```cpp
 #include <vector>
-
-using namespace std;
-
-vector<int> sieve_of_eratosthenes(int n) {
-    vector<bool> is_prime(n + 1, true);
-    vector<int> primes;
+std::vector<int> sieve_of_eratosthenes(int n) {
+    std::vector<bool> is_prime(n + 1, true);
+    std::vector<int> primes;
     is_prime[0] = is_prime[1] = false;
     for (int i = 2; i * i <= n; ++i) {
         if (is_prime[i]) {