Update index.mdx

Replace broken internal link, fix a typo in that link and change paragraph on immutable Strings to avoid confusion

Author: twisst

content/tutorials/text/strings-and-drawing-text/index.mdx

@@ -63,13 +63,13 @@ println(message.length());
 We can also change a String to all uppercase using the [toUpperCase()](http://processing.org/reference/String_toUpperCase_.html) method ([toLowerCase()](http://processing.org/reference/String_toLowerCase_.html) is also available).
 
 ```
-String uppercase = message.toUpperCase();
-println(uppercase);
+message = message.toUpperCase();
+println(message);
 ```
 
-You might notice something a bit odd here. Why didn't we simply say `message.toUpperCase()` and then print `message` variable? Instead, we assigned the result of `message.toUpperCase()` to a new variable with a different name—`uppercase`.
+You might notice something a bit odd here. Why didn't we simply say `message.toUpperCase()` and then print `message` variable? Instead, we set the variable `message` to be the result of `message.toUpperCase()`.
 
-This is because a String is a special kind of object. It is immutable. An immutable object is one whose data can never be changed. Once we create a String, it stays the same for life. Anytime we want to change the String, we have to create a new one. So in the case of converting to uppercase, the method `toUpperCase()` returns a copy of the String object with all caps.
+This is because a String is a special kind of object. It is immutable. An immutable object is one whose data can never be changed. Once we create a String, it stays the same for life. Anytime we want to change the String, we have to create a new one. So in the case of converting to uppercase, the method `toUpperCase()` cannot modify the original String in `message`—it only returns a copy of the String object with all caps. We can however use the existing variable `message` to store the new String object.
 
 Finally, let's look at [equals()](http://processing.org/reference/String_equals_.html). Now, Strings can be compared with the `==` operator as follows:
 
@@ -503,7 +503,7 @@ class Letter {
 }
 ```
 
-The character by character method also allows us to display text along a curve. Before we move on to letters, let's first look at how we would draw a series of boxes along a curve. This example makes heavy use of [Trignometry](http://www.processing.org/learning/trig/).
+The character by character method also allows us to display text along a curve. Before we move on to letters, let's first look at how we would draw a series of boxes along a curve. This example makes heavy use of [trigonometry](https://processing.org/reference/#math-trigonometry).
 
 [Example: Boxes along a curve](http://learningprocessing.com/examples/chp17/example-17-07-boxesoncurve)