Fix nullability explanation (#2856)

This change fixes the grammar and updates in the nullability concept and related exercise to improve consistency.

Co-authored-by: Kah Goh <[email protected]>

concepts/nullability/about.md

@@ -2,11 +2,11 @@
 
 In Java, the [`null` literal][null-keyword] is used to denote the absence of a value.
 
-[Primitive variables][primitive-data-types] in java all have a default value and therefore can never be `null`.
+[Primitive data types][primitive-data-types] in Java all have a default value and therefore can never be `null`.
 By convention, they start with a lowercase letter e.g `int`.
 
-[Reference variables][reference-data-types] contain the memory address of an object and can have a value of null.
-These variables usually start with an uppercase e.g `String`.
+[Reference types][reference-data-types] contain the memory address of an object and can have a value of `null`.
+They generally start with an uppercase letter, e.g. `String`.
 
 Attempting to assign a primitive variable a value of `null` will result in a compile time error as the variable always holds a default value of the type assigned.
 

concepts/nullability/introduction.md

@@ -2,11 +2,11 @@
 
 In Java, the `null` literal is used to denote the absence of a value.
 
-Primitive data types in java all have a default value and therefore can never be `null`.
+Primitive data types in Java all have a default value and therefore can never be `null`.
 By convention, they start with a lowercase letter e.g `int`.
 
-Reference types contain the memory address of an object can have a value of null.
-These variables usually start with an uppercase e.g `String`.
+Reference types contain the memory address of an object and can have a value of `null`.
+They generally start with an uppercase letter, e.g. `String`.
 
 Attempting to assign a primitive variable a value of `null` will result in a compile time error as the variable always holds a primitive value of the type assigned.
 

exercises/concept/tim-from-marketing/.docs/introduction.md

@@ -4,11 +4,11 @@
 
 In Java, the `null` literal is used to denote the absence of a value.
 
-Primitive data types in java all have a default value and therefore can never be `null`.
+Primitive data types in Java all have a default value and therefore can never be `null`.
 By convention, they start with a lowercase letter e.g `int`.
 
-Reference types contain the memory address of an object can have a value of null.
-These variables usually start with an uppercase e.g `String`.
+Reference types contain the memory address of an object and can have a value of `null`.
+They generally start with an uppercase letter, e.g. `String`.
 
 Attempting to assign a primitive variable a value of `null` will result in a compile time error as the variable always holds a primitive value of the type assigned.