docs: harmonize the style of bindable-function descriptions

doc/nano.texi

@@ -1150,7 +1150,7 @@ Pastes the currently stored text into the current buffer at the
 current cursor position.
 
 @item zap
-Throw away the current line (or the marked region).
+Throws away the current line (or the marked region).
 (This function is bound by default to <Meta+Delete>.)
 
 @item cutwordleft

doc/nanorc.5

@@ -507,7 +507,7 @@ Pastes the currently stored text into the current buffer at the
 current cursor position.
 .TP
 .B zap
-Throw away the current line (or the marked region).
+Throws away the current line (or the marked region).
 (This function is bound by default to <Meta+Delete>.)
 .TP
 .B cutwordleft