doc/filters: fix syntax and typo for the afade curve option

doc/filters.texi

@@ -587,18 +587,30 @@ the output audio will be silence. Default is 0.
 If set this option is used instead of @var{nb_samples} one.
 
 @item curve
-Set cuve for fade transition.
+Set curve for fade transition.
+
+It accepts the following values:
 @table @option
-@item @var{tri}  (triangular, linear slope (default))
-@item @var{qsin} (quarter of sine wave)
-@item @var{hsin} (half of sine wave)
-@item @var{esin} (exponential sine wave)
-@item @var{log}  (logarithmic)
-@item @var{par}  (inverted parabola)
-@item @var{qua}  (quadratic)
-@item @var{cub}  (cubic)
-@item @var{squ}  (square root)
-@item @var{cbr}  (cubic root)
+@item tri
+select triangular, linear slope (default)
+@item qsin
+select quarter of sine wave
+@item hsin
+select half of sine wave
+@item esin
+select exponential sine wave
+@item log
+select logarithmic
+@item par
+select inverted parabola
+@item qua
+select quadratic
+@item cub
+select cubic
+@item squ
+select square root
+@item cbr
+select cubic root
 @end table
 @end table