Fix typo. Uppercase first word in sentence.

Author: barik

pages/Advanced Types.md

@@ -525,7 +525,7 @@ declare function aliased(arg: Alias): Alias;
 declare function interfaced(arg: Interface): Interface;
 ```
 
-in older versions of TypeScript, type aliases couldn't be extended or implemented from (nor could they extend/implement other types). As of version 2.7, type aliases can be extended by creating a new intersection type e.g. `type Cat = Animal & { purrs: true }`.
+In older versions of TypeScript, type aliases couldn't be extended or implemented from (nor could they extend/implement other types). As of version 2.7, type aliases can be extended by creating a new intersection type e.g. `type Cat = Animal & { purrs: true }`.
 
 Because [an ideal property of software is being open to extension](https://en.wikipedia.org/wiki/Open/closed_principle), you should always use an interface over a type alias if possible.