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

Suggested by G. Brandon Robinson.

man/man1/htmlroff.1

@@ -21,7 +21,7 @@ htmlroff \- HTML formatting and typesetting
 .SH DESCRIPTION
 .I Htmlroff
 accepts
-.IM troff (1)
+.MR troff (1)
 input in the named 
 .I files
 and formats it as HTML for viewing in a web browser.
@@ -63,7 +63,7 @@ HTML entity sequences
 and so on).
 .I Htmlroff
 invokes
-.IM tcs (1)
+.MR tcs (1)
 for the conversion.
 .TP
 .B -v
@@ -73,7 +73,7 @@ Generate debugging output and warnings about suspicious input.
 Most 
 .I troff
 input files, especially those using the
-.IM ms (7)
+.MR ms (7)
 macros, can be used unaltered.
 In general, the macro file
 .B tmac.html
@@ -83,10 +83,10 @@ as in
 .B -ms
 .BR -mhtml .
 .PP
-.IM Htmlroff (7)
+.MR Htmlroff (7)
 describes the changes to the input language.
 .PP
-.IM Mhtml (7)
+.MR Mhtml (7)
 describes the new macros.
 .SH EXAMPLES
 Format the Plan 9 web page:
@@ -113,7 +113,7 @@ to Unicode characters like α.
 .SH SOURCE
 .B \*9/src/cmd/htmlroff
 .SH "SEE ALSO
-.IM tcs (1) ,
-.IM troff (1) ,
-.IM htmlroff (7) ,
-.IM mhtml (7)
+.MR tcs (1) ,
+.MR troff (1) ,
+.MR htmlroff (7) ,
+.MR mhtml (7)