Update README.md

Author: 5cript

README.md

@@ -158,12 +158,12 @@ Returns whether or not the tree is empty.
 ### iterator begin()
 Returns the iterator of the interval with the lowest lower_bound.
 
-**Returns**: Is this tree empty?
+**Returns**: begin iterator.
 
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 ### iterator end()
 Returns a past the end iterator.
 
-**Returns**: Is this tree empty?
+**Returns**: past the end iterator.
 
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