guide.md: update 'Selection Challenges' section

Pull Request: #337

Author: NPalopoli

_extras/guide.md

@@ -162,11 +162,12 @@ Tip: use `.head()` method throughout this lesson to keep your display neater for
 
 ### Selection Challenges
 
-* What happens when you type:
+* What happens when you execute:
 
-	`surveys_df[0:3]`  
-	`surveys_df[:5]` slicing from first element makes 0 redundant  
-	`surveys_df[-1:]` you can count backwards  
+	`surveys_df[0:3]`
+	`surveys_df[0:1]` slicing only the first element
+	`surveys_df[:5]` slicing from first element makes 0 redundant
+	`surveys_df[-1:]` you can count backwards
 
 	*Suggestion*: You can also select every Nth row: `surveys_df[1:10:2]`. So, how to interpret `surveys_df[::-1]`?