Update README.md

Author: czekster

challenges/README.md

@@ -4,7 +4,7 @@ I present two projects:
 - `project00`: individual challenges (from 01 to 07), each with its own `main()` function, `defs.h` and `Makefile` files (you may build individual challenges, e.g., `make challenge06`), and `markov.c` with the implementations.
 - `project01`: a full project (with a single `main()` function), with lots of 'services', such as:
 
-<pre>
+```
 void echo_help() {
    printf("This is the Markov chain solver [last update: 23/03/2022].\n");
    printf("To run DTMC: markov <FILE> <OP>\n");
@@ -15,4 +15,4 @@ void echo_help() {
    printf("   3 Convert CTMC to Embedded DTMC\n");
    printf("   4 Simulate CTMC (pass <TIME> parameter!)\n");
 }
-</pre>
+```