tmac: rename IM (italic manual) to MR (manual reference)

Suggested by G. Brandon Robinson.

man/man3/cachechars.3

@@ -30,7 +30,7 @@ A
 may contain too many characters to hold in memory
 simultaneously.
 The graphics library and draw device (see
-.IM draw (3) )
+.MR draw (3) )
 cooperate to solve this problem by maintaining a cache of recently used
 character images.
 The details of this cooperation need not be known by most programs:
@@ -127,7 +127,7 @@ A
 .B Font
 consists of an overall height and ascent
 and a collection of subfonts together with the ranges of runes (see
-.IM utf (7) )
+.MR utf (7) )
 they represent.
 Fonts are described by the following structures.
 .IP
@@ -181,7 +181,7 @@ The
 and
 .LR ascent
 fields of Font are described in
-.IM graphics (3) .
+.MR graphics (3) .
 .L Sub
 contains
 .L nsub
@@ -302,12 +302,12 @@ for replacement when the cache is full.
 .SH SOURCE
 .B \*9/src/libdraw
 .SH SEE ALSO
-.IM graphics (3) ,
-.IM allocimage (3) ,
-.IM draw (3) ,
-.IM subfont (3) ,
-.IM image (7) ,
-.IM font (7)
+.MR graphics (3) ,
+.MR allocimage (3) ,
+.MR draw (3) ,
+.MR subfont (3) ,
+.MR image (7) ,
+.MR font (7)
 .SH DIAGNOSTICS
 All of the functions use the graphics error function (see
-.IM graphics (3) ).
+.MR graphics (3) ).