fix: reword error message

maps.md

@@ -551,7 +551,7 @@ We added our own error type and are returning a `nil` error.
 With these changes, we now get a very clear error:
 
 ```
-dictionary_test.go:66: got error '%!q(<nil>)' want 'cannot update word because it does not exist'
+dictionary_test.go:66: got error '%!q(<nil>)' want 'cannot perform operation on word because it does not exist'
 ```
 
 ## Write enough code to make it pass