Fix typo

This applies thejameskyle/the-super-tiny-compiler@c5979db0260eb6784321c5231a20ca3b9ac44522

0-introduction.md

@@ -213,7 +213,7 @@ var visitor = {
 };
 ```
 
-However, there also exists the possibilty of calling things on "exit". Imagine
+However, there also exists the possibility of calling things on "exit". Imagine
 our tree structure from before in list form:
 
 - Program