Minor tweaks based on review comments

Author: frank-at-adacore

courses/rust_essentials/110_common_library_types/02-option.rst

@@ -15,7 +15,7 @@ Why Use "Option<T>"?
 
   * Which can be difficult to find
 
-* Variable is *always* its type
+* In Rust, a variable is *always* its type
 
   * So the value is always valid
   * To allow for *unknown* we wrap the type in :rust:`Option`

courses/rust_essentials/180_modules/01-modules.rst

@@ -8,7 +8,7 @@ Big Picture
 
 * Most applications reside in more than one file
 
-  * *Modules* are how to organize code
+  * Rust uses *modules* to organize code
 
 * Encapsulation