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

Suggested by G. Brandon Robinson.

man/man7/ms.7

@@ -18,7 +18,7 @@ ms \- macros for formatting manuscripts
 This package of
 .I nroff
 and
-.IM troff (1)
+.MR troff (1)
 macro definitions provides a canned formatting
 facility for tech%nical papers in various formats.
 .PP
@@ -38,11 +38,11 @@ impunity after the first
 .LR .na .
 .PP
 Output of the
-.IM eqn (1) ,
-.IM tbl (1) ,
-.IM pic (1)
+.MR eqn (1) ,
+.MR tbl (1) ,
+.MR pic (1)
 and
-.IM grap (1)
+.MR grap (1)
 preprocessors
 for equations, tables, pictures, and graphs is acceptable as input.
 .SH FILES
@@ -57,8 +57,8 @@ Tenth Edition, Volume 2.
 .br
 .IR eqn (1), 
 .IR troff (1), 
-.IM tbl (1) ,
-.IM pic (1)
+.MR tbl (1) ,
+.MR pic (1)
 .SH REQUESTS
 .ta \w'..ND \fIdate\fR 'u +\w'Initial 'u +\w'Cause 'u
 .br
@@ -153,7 +153,7 @@ Implies
 produced by
 .I neqn
 or
-.IM eqn (1) .
+.MR eqn (1) .
 .ti0
 \fL\&.EQ\fP \fIx y\fR	-	yes	Display equation.
 Equation number is
@@ -234,7 +234,7 @@ is subsection level (default 1).
 \fL\&.P2\fP	-	yes	End program display.
 .ti0
 \fL\&.PE\fP	-	yes	End picture; see
-.IM pic (1) .
+.MR pic (1) .
 .ti0
 \fL\&.PF\fP	-	yes	End picture; restore vertical
 position.
@@ -280,7 +280,7 @@ font automatically bold.
 Default is 5 10 15 ...
 .ti0
 \fL\&.TE\fP	-	yes	End table; see
-.IM tbl (1) .
+.MR tbl (1) .
 .ti0
 \fL\&.TH\fP	-	yes	End heading section of table.
 .ti0