use hash for comment.

using a hash mark for the comment makes it slightly more 
convenient for the reader by allowing them to copy and paste
directly into the terminal.

Author: switch-to-gitlab

book/src/01_calculator/calc_intro.md

@@ -7,7 +7,7 @@ If you haven't cloned the [GitHub](https://github.com/ehsanmok/create-your-own-l
 To start, we have `1 + 1;` in [examples/simple.calc](https://github.com/ehsanmok/create-your-own-lang-with-rust/blob/master/calculator/examples/simple.calc) where you can compile with
 
 ```text
-cargo build --bin main // create a simple executable for Calc
+cargo build --bin main # create a simple executable for Calc
 ../target/debug/main examples/simple.calc
 ```