pangram: Clarify instructions

Unfortunately #2215 introduced an ambiguity for some downstream implementations of this exercise that use non-ASCII inputs that shouldn't be considered part of the alphabet for the purpose of defining pangrams.

This PR is meant to clarify that only 'a':'z' are relevant to determine if a sentence is a pangram without restricting the inputs to those characters.

See also: exercism/julia#614

Author: SaschaMann

exercises/pangram/instructions.md

@@ -5,4 +5,4 @@ Your task is to figure out if a sentence is a pangram.
 A pangram is a sentence using every letter of the alphabet at least once.
 It is case insensitive, so it doesn't matter if a letter is lower-case (e.g. `k`) or upper-case (e.g. `K`).
 
-For this exercise we only use the basic letters used in the English alphabet: `a` to `z`.
+For this exercise we only consider the basic letters used in English as part of the alphabet: `a` to `z`.