doc/filters: fix typo in eq contrast option

Fixes ticket #4866.
Signed-off-by: Lou Logan <lou@lrcd.com>

doc/filters.texi

@@ -5309,7 +5309,7 @@ The filter accepts the following options:
 @table @option
 @item contrast
 Set the contrast expression. The value must be a float value in range
-@code{-2.0} to @code{2.0}. The default value is "0".
+@code{-2.0} to @code{2.0}. The default value is "1".
 
 @item brightness
 Set the brightness expression. The value must be a float value in