fix #H7354 - incorrect material

for parchment and vellum spellbooks.  Parchment and vellum are made
from animal skin rather than from plants, so classifying spellbooks
with those descriptions as paper is inaccurate.  Changing them to be
made out of leather has a couple of side-effects:  eating them while
polymorphed will break vegetarian conduct and polymorphing them might
result in a leather golem rather than a paper one.

I left "leathery spell book" as paper since that directly refers to
the cover.  The composition shouldn't be changed--the pages of such
a book are still made out of paper--but the effect of eating one
possibly should.  (That description originally was "leather" and got
changed.  I don't remember the details and assumed it was due to odd
wishing behavior, but there's a commented-out routine in eat.c which
suggests it was related to eating instead.  Anyway, "leathery" is
still non-leather.)

doc/fixes36.2

@@ -96,6 +96,9 @@ object scattering during shop wall repair was skipped if a trap at the same
 augmented death reason for "while helpless" was broken for record and logfile
 	(but still correct for xlogfile)
 prevent wish prompt input from remembering the previous wish
+parchment and vellum are made from animal skin so change material composition
+	and color for spellbooks with those descriptions from paper to leather;
+	eating those books now breaks vegetarian conduct
 
 
 Fixes to Post-3.6.1 Problems that Were Exposed Via git Repository