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

Suggested by G. Brandon Robinson.

man/man3/searchpath.3

@@ -13,9 +13,9 @@ char*	searchpath(char *name)
 searches for the executable
 .I name
 in the same way that
-.IM sh (1)
+.MR sh (1)
 and
-.IM rc (1)
+.MR rc (1)
 do.
 .PP
 The environment variable
@@ -32,9 +32,9 @@ returns a pointer to a malloced string containing a path
 or simply
 .IR name )
 suitable for use in 
-.IM open (3)
+.MR open (3)
 or 
-.IM exec (3) .
+.MR exec (3) .
 .PP
 If
 .I name