cat: Use present tense

Event: Advanced UNIX Programming Course (Fall23) at NTHU.
Reviewed by: imp, lwhsu
Pull Request: freebsd#923

bin/cat/cat.1

@@ -152,7 +152,7 @@ character, print the contents of
 read and output contents of the standard input again, then finally output
 the contents of
 .Pa file3 .
-Note that if the standard input referred to a file, the second dash
+Note that if the standard input refers to a file, the second dash
 on the command-line would have no effect, since the entire contents of the file
 would have already been read and printed by
 .Nm