Fix grammar and clarity in reference types explanation

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

@@ -7,8 +7,8 @@ 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`.
 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. 
+These variables usually 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.