fix the name of argument to ftplib.FTP.set_pasv and fix wording (GH-653) (GH-654)

diff --git a/Doc/library/ftplib.rst b/Doc/library/ftplib.rst
index b8c1dcf..1153ce7 100644
--- a/Doc/library/ftplib.rst
+++ b/Doc/library/ftplib.rst
@@ -255,9 +255,9 @@
    prints the line to ``sys.stdout``.
 
 
-.. method:: FTP.set_pasv(boolean)
+.. method:: FTP.set_pasv(val)
 
-   Enable "passive" mode if *boolean* is true, other disable passive mode.
+   Enable "passive" mode if *val* is true, otherwise disable passive mode.
    Passive mode is on by default.