execve.2: Minor clarification

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man2/execve.2

@@ -63,7 +63,8 @@ By convention, the first of these strings (i.e.,
 should contain the filename associated with the file being executed.
 \fIenvp\fP is an array of strings, conventionally of the form
 \fBkey=value\fP, which are passed as environment to the new program.
-Both \fIargv\fP and \fIenvp\fP must be terminated by a null pointer.
+The \fIargv\fP and \fIenvp\fP arrays must each include a null pointer
+at the end of the array.
 
 The argument vector and environment can be accessed by the
 called program's main function, when it is defined as: