Fix modules notes and formatting

Rust/Cargo.md

@@ -80,7 +80,9 @@ crate::module::function();  // abs path (same crate)
 
 ### Public vs Private
 
-Modules aren’t useful only for organizing the code. They also define Rust’s privacy boundary: the line that encapsulates the implementation details external code isn’t allowed to know about, call, or rely on. So, to make an item like a function or struct private, put it in a module.
+Modules aren’t useful only for organizing the code. They also define Rust’s privacy boundary:  
+the line that encapsulates the implementation details external code isn’t allowed to know about, call, or rely on.  
+So, to make an item like a function or struct private, put it in a module.
 
 The way privacy works in Rust is that all items (functions, methods, structs, enums, modules, and constants) are *private by default*.  
 Items in a parent module can’t use the private items inside child modules, but items in child modules can use the items in their ancestor modules.  
@@ -100,7 +102,8 @@ mod file_level_module;  // define a module for the whole file (same name as file
 ```
 
 It's possible to use `pub` to designate *structs* and *enums* as public, but there are a few extra details.  
-If `pub` is used before a struct definition, this makes the struct public, but the struct’s fields will still be private. It's possible make each field public or not on a case-by-case basis.
+If `pub` is used before a struct definition, this makes the struct public, but the struct’s fields will still be private.  
+It's possible make each field public or not on a case-by-case basis.
 
 In contrast, if an enum is made public, all of its variants are then public.
 
@@ -108,8 +111,8 @@ In contrast, if an enum is made public, all of its variants are then public.
 
 ```rs
 use <crate_name>::module;  // import module (abs path, other crate)
-use crate::module;  // s (abs path, same crate)
+use crate::module;  // import module (abs path, same crate)
-use self::module;  // // import module (rel path, same crate)
+use self::module;  // import module (rel path, same crate)
 
 use <crate_name>::module as alias;  // import module w/ aliass
 pub use <crate_name>::module;  // re-exporting (import and make available to others)
@@ -124,9 +127,17 @@ module::function();  // use func w/ shorter path
 
 ```txt
 src
-|_main.rs  --> app entrypoint
+|_main.rs  --> default executable file
-|_lib.rs  --> export module-folders
+|_lib.rs  --> default library file
 |__module
-|  |_mod.rs --> export module-files
+|  |_mod.rs --> export submodules
-|  |_file.rs
+|  |_submodule.rs --> submodule
+```
+
+```rs
+// main.rs
+mod module;  // delcare module direcotry as a module
+
+// mod.rs
+pub mod sub_module;  // declare sub_module file as a module
 ```