Update C manual memory management example (#902)

In C (as opposed to C++) the explicit cast from void* to int* is not required. It is also not idiomatic to do so in C code.

Actual C codebase would use `malloc()` without the cast, and a C++ one (when not using abstractions) a `new int[n]` - both a bit cleaner and less verbose than this example.

src/memory-management/manual.md

@@ -10,7 +10,7 @@ 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 = malloc(n * sizeof(int));
     //
     // ... lots of code
     //