avfilter/vf_libvmaf: Update ssim, ms_ssim options description

Update description for ssim and ms_ssim libvmaf options to specify
feature=float_ssim and feature=float_ms_ssim which are used to request
ssim and ms_ssim values in the latest versions of libvmaf.

Signed-off-by: Yondon Fu <yondon.fu@gmail.com>

libavfilter/vf_libvmaf.c

@@ -70,8 +70,8 @@ static const AVOption libvmaf_options[] = {
     {"enable_transform",  "use model='enable_transform=true'.",                         OFFSET(enable_transform), AV_OPT_TYPE_BOOL, {.i64=0}, 0, 1, FLAGS|AV_OPT_FLAG_DEPRECATED},
     {"phone_model",  "use model='enable_transform=true'.",                              OFFSET(phone_model), AV_OPT_TYPE_BOOL, {.i64=0}, 0, 1, FLAGS|AV_OPT_FLAG_DEPRECATED},
     {"psnr",  "use feature='name=psnr'.",                                               OFFSET(psnr), AV_OPT_TYPE_BOOL, {.i64=0}, 0, 1, FLAGS|AV_OPT_FLAG_DEPRECATED},
-    {"ssim",  "use feature='name=ssim'.",                                               OFFSET(ssim), AV_OPT_TYPE_BOOL, {.i64=0}, 0, 1, FLAGS|AV_OPT_FLAG_DEPRECATED},
-    {"ms_ssim",  "use feature='name=ms_ssim'.",                                         OFFSET(ms_ssim), AV_OPT_TYPE_BOOL, {.i64=0}, 0, 1, FLAGS|AV_OPT_FLAG_DEPRECATED},
+    {"ssim",  "use feature='name=float_ssim'.",                                         OFFSET(ssim), AV_OPT_TYPE_BOOL, {.i64=0}, 0, 1, FLAGS|AV_OPT_FLAG_DEPRECATED},
+    {"ms_ssim",  "use feature='name=float_ms_ssim'.",                                   OFFSET(ms_ssim), AV_OPT_TYPE_BOOL, {.i64=0}, 0, 1, FLAGS|AV_OPT_FLAG_DEPRECATED},
     {"pool",  "Set the pool method to be used for computing vmaf.",                     OFFSET(pool), AV_OPT_TYPE_STRING, {.str=NULL}, 0, 1, FLAGS},
     {"n_threads", "Set number of threads to be used when computing vmaf.",              OFFSET(n_threads), AV_OPT_TYPE_INT, {.i64=0}, 0, UINT_MAX, FLAGS},
     {"n_subsample", "Set interval for frame subsampling used when computing vmaf.",     OFFSET(n_subsample), AV_OPT_TYPE_INT, {.i64=1}, 1, UINT_MAX, FLAGS},