Update types.md (#1199)

Fix typo

standard/types.md

@@ -725,7 +725,7 @@ A *nullable reference type* is denoted by appending a `?` to a valid non-nullabl
 There are two forms of nullability for reference types:
 
 - *nullable*: A *nullable-reference-type* can be assigned `null`. Its default null state is *maybe-null*.
-- *non-nullable*” A *non-nullable reference* should not be assigned a `null` value. Its default null state is *not-null*.
+- *non-nullable*: A *non-nullable reference* should not be assigned a `null` value. Its default null state is *not-null*.
 
 > *Note:* The types `R` and `R?` are represented by the same underlying type, `R`. A variable of that underlying type can either contain a reference to an object or be the value `null`, which indicates “no reference.” *end note*