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

Suggested by G. Brandon Robinson.

man/man1/sed.1

@@ -78,7 +78,7 @@ that
 addresses the last line of input, or a context address,
 .BI / regular-expression / \f1,
 in the style of
-.IM regexp (7) ,
+.MR regexp (7) ,
 with the added convention that
 .L \en
 matches a
@@ -235,7 +235,7 @@ in the pattern space.
 Any character may be used instead of 
 .LR / .
 For a fuller description see
-.IM regexp (7) .
+.MR regexp (7) .
 .I Flags
 is zero or more of
 .RS
@@ -374,8 +374,8 @@ formatted manuscript.
 .IR grep (1), 
 .IR awk (1), 
 .IR lex (1), 
-.IM sam (1) ,
-.IM regexp (7)
+.MR sam (1) ,
+.MR regexp (7)
 .br
 L. E. McMahon,
 `SED \(em A Non-interactive Text Editor',