Replace "zero byte" with "null byte" as per man-pages style guide.

docs/pv.1

@@ -413,7 +413,7 @@ pipes or non-regular files, or are unreadable, the total size will not be
 calculated.
 .TP
 .B \-0, \-\-null
-Count lines as terminated with a zero byte instead of with a newline.
+Count lines as terminated with a null byte instead of with a newline.
 This option implies \-\-line\-mode.
 .TP
 .BI \-i\  SEC \fR,\ \fB\-\-interval\  SEC

docs/pv.1.md

@@ -256,7 +256,7 @@ display types that are explicitly switched on will be shown.
 
 **-0, \--null**
 
-:   Count lines as terminated with a zero byte instead of with a
+:   Count lines as terminated with a null byte instead of with a
     newline. This option implies \--line-mode.
 
 **-i SEC, \--interval SEC**