doc/filters: fix @item syntax in showspectrum docs

In particular, fix warnings:
** Unknown command with braces `@item' (in doc/filters.texi l. 60XX)

doc/filters.texi

@@ -6068,13 +6068,13 @@ Specify scale used for calculating intensity color values.
 
 It accepts the following values:
 @table @option
-@item{lin}
+@item lin
 linear
-@item{sqrt}
+@item sqrt
 square root, default
-@item{cbrt}
+@item cbrt
 cubic root
-@item{log}
+@item log
 logarithmic
 @end table