doc: make formatting of SEE ALSO in cat.1 and tac.1 consistent

None of the coreutils man pages - but the two above - are using bold
setting for the references to other man pages in the SEE ALSO section.

* man/cat.x (SEE ALSO): Remove '\fB...\fP' setting.
* man/tac.x: Likewise, and add a reference to cat(1).

man/cat.x

@@ -3,4 +3,4 @@ cat \- concatenate files and print on the standard output
 [DESCRIPTION]
 .\" Add any additional description here
 [SEE ALSO]
-\fBtac\fP(1)
+tac(1)

man/tac.x

@@ -3,4 +3,4 @@ tac \- concatenate and print files in reverse
 [DESCRIPTION]
 .\" Add any additional description here
 [SEE ALSO]
-\fBrev\fP(1)
+cat(1), rev(1)