Fix AS Migration Guide upcasting formatting (#70)

Author: evaporei

pages/en/developer/assemblyscript-migration-guide.mdx

@@ -180,7 +180,8 @@ let c: usize = a + (b as usize)
 // upcasting on class inheritance
 class Bytes extends Uint8Array {}
 
-let bytes = new Bytes(2) < Uint8Array > bytes // same as: bytes as Uint8Array
+let bytes = new Bytes(2)
+// <Uint8Array>bytes // same as: bytes as Uint8Array
 ```
 
 There are two scenarios where you may want to cast, but using `as`/`<T>var` **isn't safe**:
@@ -192,15 +193,17 @@ There are two scenarios where you may want to cast, but using `as`/`<T>var` **is
 // downcasting on class inheritance
 class Bytes extends Uint8Array {}
 
-let uint8Array = new Uint8Array(2) < Bytes > uint8Array // breaks in runtime :(
+let uint8Array = new Uint8Array(2)
+// <Bytes>uint8Array // breaks in runtime :(
 ```
 
 ```typescript
 // between two types that share a superclass
 class Bytes extends Uint8Array {}
 class ByteArray extends Uint8Array {}
 
-let bytes = new Bytes(2) < ByteArray > bytes // breaks in runtime :(
+let bytes = new Bytes(2)
+// <ByteArray>bytes // breaks in runtime :(
 ```
 
 For those cases, you can use the `changetype<T>` function: