Phone Number: Changed wording of the additional instruction (#467)

The following changes have been made, per BNAndras' suggestions.

- Each sentence is now on a separate line.

- The phrase "user-entered phone number(s)" has been replaced with
"phone numbers".

- The word "input" has been repaced with "parameter".

Author: borderite

exercises/practice/phone-number/.docs/instructions.append.md

@@ -1,6 +1,10 @@
 # Instructions Append
 
-In addition to the function, _numbers_, that cleans up user-entered phone numbers, you need to write two more functions, _area-code_ and _pprint_ in the Emacs Lisp track. Both of the extra functions take the user-entered phone number as their inputs. The first function _area-code_ extracts the area code from the input, while the second function _pprint_ transforms the input to a string in the format of 
+In addition to the function, _numbers_, that cleans up phone numbers, you need to write two more functions, _area-code_ and _pprint_ in the Emacs Lisp track. 
+
+Each of the extra functions takes the phone number as its only parameter.
+
+The first function _area-code_ extracts the area code from the input, while the second function _pprint_ transforms the input to a string in the format of 
 ```
 (NXX) NXX-XXXX
 ```