series: fix typo in instructions (#1922)

exercises/practice/series/.docs/instructions.append.md

@@ -7,4 +7,4 @@ Consider the advantages and disadvantages of the following possibilities:
 - Crash the program with `panic!`.
 - Return a `Result::Err`. (not possible here, because the function signature is given)
 - Return an empty vector.
-- Return a vector containing as many empty strings as the lenght of the string "digits" **plus one**. (this has some nice mathematical properties!)
+- Return a vector containing as many empty strings as the length of the string "digits" **plus one**. (this has some nice mathematical properties!)