Correct grammar in instructions.append.md

exercises/practice/grade-school/.docs/instructions.append.md

@@ -1,5 +1,5 @@
 # Instructions append
 
-It isn't strictly necessary to model students as a seperate object, you can use a convenient internal data strcture as the requirements are quite simple (and as long as it doesn't leak out into the results).
+It isn't strictly necessary to model students as a separate object, you can use a convenient internal data structure as the requirements are quite simple (and as long as it doesn't leak out into the results).
 
 Note: this exercise has been slightly modified from the problem-specification, as it makes more sense to separate adding students and querying them in separate methods (vs. having one method doing both, which feels less Pharo/Smalltalk like).