Fix typos/grammar/code examples

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() {

src/strings_ii/check_if_blank.md

@@ -2,8 +2,7 @@
 
 You can check if a `String` is blank by using the `.isBlank` method.
 
-The difference is that an empty `String` has actually zero characters. A blank `String`
-is can have characters, so long as those characters are what we would consider whitespace.
+The difference is that an empty `String` has actually zero characters. A blank `String` can have characters, so long as those characters are what we would consider whitespace.
 That is, things like spaces and newlines.
 
 ```java

src/switch/exhaustiveness.md

@@ -45,5 +45,6 @@ boolean isScary(Bird bird) {
             return false;
         }
     }
+    return false;
 }
 ```