Fix C array syntax in memory-management/manual.md

Heap-allocated arrays in C use `int*` instead of `int[]`.

src/memory-management/manual.md

@@ -8,7 +8,7 @@ You must call `free` on every pointer you allocate with `malloc`:
 
 ```c
 void foo(size_t n) {
-    int[] int_array = (int*)malloc(n * sizeof(int));
+    int* int_array = (int*)malloc(n * sizeof(int));
     //
     // ... lots of code
     //