Better documented changes to doc/specs/stdlib_sorting.md

I got confused and in my commit of stdlib_sorting.md thought I was committing
stdlib_sorting.fypp. The commit to stdlib_sorting.md actualy documented the
changes in the output of test_sorting.f90, i.e., the reduction in the size of
the arrays being sorted, so the runtime of test_sorting.f90 would be less than
1 second.

[ticket: X]

Author: wclodius2

doc/specs/stdlib_sorting.md

@@ -493,7 +493,7 @@ integer arrays each of size `2**16`:
   replaced by random values.
 
 On three different default character arrays, each of length 4 and of
-size `20**4, with characters drawn from the set a-p:
+size `20**4, with characters drawn from the set "a"-"p":
 
 * Char. Decreasing - values decrease uniformly from `"pppp"` to
   `"aaaa"`.
@@ -503,7 +503,7 @@ size `20**4, with characters drawn from the set a-p:
   random order.
 
 On three different `string_type` arrays, each of length 4 elements and
-of size `16**3`, with characters drawn from the set a-p:
+of size `16**3`, with characters drawn from the set "a"-"p":
 
 * String Decreasing - values decrease uniformly from `"ppp"` to
   `"aaa"`.