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

Suggested by G. Brandon Robinson.

man/man1/src.1

@@ -19,11 +19,11 @@ examines the named
 to find the corresponding source code, which is then sent to the editor using
 .B B
 (see
-.IM sam (1) ).
+.MR sam (1) ).
 If
 .I file
 is an
-.IM rc (1)
+.MR rc (1)
 script, the source is the file itself.
 If
 .I file
@@ -35,7 +35,7 @@ and
 will point the editor at the line that begins the definition.
 .I Src
 uses
-.IM db (1)
+.MR db (1)
 to extract the symbol table information that identifies the source.
 .PP
 .I Src
@@ -78,6 +78,6 @@ src -s strcmp rc
 .SH SOURCE
 .B \*9/bin/src
 .SH "SEE ALSO"
-.IM db (1) ,
-.IM plumb (1) ,
-.IM sam (1) .
+.MR db (1) ,
+.MR plumb (1) ,
+.MR sam (1) .