fix: make hello public

Author: Satya Rohith

src/doc/rustc/src/what-is-rustc.md

@@ -50,7 +50,7 @@ fn main() {
 And a `foo.rs` that had this:
 
 ```rust,ignore
-fn hello() {
+pub fn hello() {
     println!("Hello, world!");
 }
 ```
@@ -65,4 +65,4 @@ No need to tell `rustc` about `foo.rs`; the `mod` statements give it
 everything that it needs. This is different than how you would use a C
 compiler, where you invoke the compiler on each file, and then link
 everything together. In other words, the *crate* is a translation unit, not a
-particular module.
+particular module.