[doc] add missing example

src/plugins/e-acsl/doc/userman/examples/instrument.i

+void f(int *p) {
+  *p = 0;
+}
+
+void g(int *p) {
+  *p = 1;
+}
+
+int main(void) {
+  int t[3];
+  f(&t[0]);
+  /*@ assert \initialized(&t[0]); */
+  t[1] = 1;
+  g(&t[1]);
+  g(&t[2]);
+  /*@ assert \initialized(&t[1]); */
+  /*@ assert \initialized(&t[2]); */
+  return 0;
+}