doc/Guidebook.mn: Simplify table format.

The 'A' ("alphabetic") and 'N' ("numeric") column classifiers were being
used to little benefit.

Since 'A' was applying to every row of the table, none was more indented
than any other, except via the inclusion of unadjustable, unbreakable
space escape sequences `\ `, which work just as well with column
classifier 'L' ("left").

In fact, even they are unnecessary; regular spaces will do.

tbl(1):
     Ordinarily, a table entry is typeset rigidly.  It is not filled,
     broken, hyphenated, adjusted, or populated with additional inter‐
     sentence space.

...so furthermore convert the escaped spaces to regular ones.

Similarly, 'N' applies several rules to manage alignment of decimal
points.  This table doesn't need them.  Right-alignment of integers is
just as easily achieved with the 'R' ("right") column classifier.

doc/Guidebook.mn

@@ -2825,15 +2825,15 @@ Here is a list of the armor class values provided by suits of armor:
 .\" line and leading spaces to indent their second line.
 .TS
 center;
-a n.
+l r.
 Dragon scale mail	1
 Plate mail, Crystal plate mail	3
 Bronze plate mail, Splint mail,	\&
-\ \ \ Banded mail, Dwarvish mithril-coat	4
+   Banded mail, Dwarvish mithril-coat	4
 Chain mail, Elven mithril-coat	5
 Scale mail, Orcish chain mail	6
 Ring mail, Studded leather armor,	\&
-\ \ \ Dragon scales	7
+   Dragon scales	7
 Leather armor, Orcish ring mail	8
 Leather jacket	9
 none	10