Remove unused .meta/description.md's (JuliaLang#509)

* Remove accumulate desc

* Remove pangram desc

* Remove perfect-numbers desc

* Integrate pythagorean-triplet desc

* Reinstate ASCII clarification in pangram desc

Author: cmcaine

exercises/practice/accumulate/.meta/description.md

@@ -5,5 +5,5 @@ Determine if a sentence is a pangram. A pangram (Greek: παν γράμμα, pan
 The best known English pangram is:
 > The quick brown fox jumps over the lazy dog.
 
-The alphabet used consists of letters `a` to `z`, inclusive, and is case
+The alphabet used consists of ASCII letters `a` to `z`, inclusive, and is case
 insensitive.

exercises/practice/pangram/.docs/instructions.md

@@ -4,7 +4,7 @@ A Pythagorean triplet is a set of three natural numbers, {a, b, c}, for
 which,
 
 ```text
-a**2 + b**2 = c**2
+a^2 + b^2 = c^2
 ```
 
 and such that,
@@ -16,7 +16,7 @@ a < b < c
 For example,
 
 ```text
-3**2 + 4**2 = 9 + 16 = 25 = 5**2.
+3^2 + 4^2 = 9 + 16 = 25 = 5^2.
 ```
 
 Given an input integer N, find all Pythagorean triplets for which `a + b + c = N`.