* Regenerate exercise READMEs to pick up tweaks to problem description

The upstream problem specifications have been updated with various
formatting changes, whitespace changes, and clarifications to the description.

This pulls in all the recent changes.

* Inline the exercise README insert

Now that we're operating with a track-wide template for generating exercise
READMEs we no longer need a separate markdown document to use for hints
and instructions that apply to all the exercises in a track. These instructions
can be defined directly in the template.

* parallel-letter-frequency: remove characters with accents / diacritics

* parallel-letter-frequency: update README with stretch goal

* parallel-letter-frequency: fix readme

Skipped Bob since the README change is for a newer version than the
current Java test suite.