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

Suggested by G. Brandon Robinson.

man/man3/mux.3

@@ -123,7 +123,7 @@ nil if an error occurred.
 .I Muxprocs
 allocates new procs 
 (see
-.IM thread (3) )
+.MR thread (3) )
 in which to run
 .I send
 and
@@ -146,7 +146,7 @@ that need to remain active.
 .I Libmux
 also provides a non-blocking interface, useful for programs forced
 to use a
-.IM select (3) -based
+.MR select (3) -based
 main loop.
 .I Muxrpcstart
 runs the first half of
@@ -176,7 +176,7 @@ with
 .SH SOURCE
 .B \*9/src/libmux
 .SH SEE ALSO
-.IM thread (3) ,
+.MR thread (3) ,
 .IR intro (9p)
 .SH BUGS
 .I Libmux