avfilter/af_superequalizer: improve description

Signed-off-by: Paul B Mahol <onemda@gmail.com>

doc/filters.texi

@@ -3836,7 +3836,7 @@ Set level of input signal of original channel. Default is 0.8.
 @end table
 
 @section superequalizer
-Apply 18th band equalizer.
+Apply 18 band equalizer.
 
 The filter accpets the following options:
 @table @option

libavfilter/af_superequalizer.c

@@ -357,7 +357,7 @@ AVFILTER_DEFINE_CLASS(superequalizer);
 
 AVFilter ff_af_superequalizer = {
     .name          = "superequalizer",
-    .description   = NULL_IF_CONFIG_SMALL("Apply 18-th band equalization filter."),
+    .description   = NULL_IF_CONFIG_SMALL("Apply 18 band equalization filter."),
     .priv_size     = sizeof(SuperEqualizerContext),
     .priv_class    = &superequalizer_class,
     .query_formats = query_formats,