Fix typos

Author: zohairawan

src/instance_methods/challenges.md

@@ -111,7 +111,7 @@ void main() {
 
 ## Challenge 4.
 
-Make a `Rectange` class which has a `width` field and a `height`
+Make a `Rectangle` class which has a `width` field and a `height`
 field. Give it an instance method named `toCharArray` which gives
 a `char[]` that can be printed to display a rectangle of the given
 width and height.

src/strings_ii/UPPERCASE.md

@@ -1,7 +1,7 @@
 # UPPERCASE
 
 Similarly, if you have a `String` which potentially contains lower-cased letters, you can get a new `String` with everything
-transformed into lower-case using the `.toUpperCase()` method.
+transformed into upper-case using the `.toUpperCase()` method.
 
 ```java
 void main() {