avifenc: refer to standard input in error message

Refer to "standard input" in the stdin Y4M error message. The outer
error already says "from standard input"; make the hint consistent.

Refs #3194

apps/avifenc.c

diff --git a/apps/avifenc.c b/apps/avifenc.c
index b396ad6..4d6106e 100644
--- a/apps/avifenc.c
+++ b/apps/avifenc.c
@@ -634,7 +634,7 @@
                 avifFileFormatToString(inputFormat),
                 currentFile->filename == AVIF_FILENAME_STDIN ? "from standard input" : currentFile->filename);
         if (currentFile->filename == AVIF_FILENAME_STDIN && inputFormat == AVIF_APP_FILE_FORMAT_Y4M) {
-            fprintf(stderr, "Specify --input-format if the input is not y4m\n");
+            fprintf(stderr, "Specify --input-format if the standard input is not Y4M\n");
         }
         return AVIF_FALSE;
     }