lavc/utils: fix a few grammar/style nits in log messages

In avcodec_open2(), apply consistent casing and avoid "or" conjunction at
the beginning of a sentence, in the messages issued in case of
experimental codec usage.

libavcodec/utils.c

@@ -933,12 +933,12 @@ int attribute_align_arg avcodec_open2(AVCodecContext *avctx, const AVCodec *code
         const char *codec_string = av_codec_is_encoder(codec) ? "encoder" : "decoder";
         AVCodec *codec2;
         av_log(NULL, AV_LOG_ERROR,
-               "%s '%s' is experimental but experimental codecs are not enabled, "
-               "Add '-strict %d' if you want to use it.\n",
+               "The %s '%s' is experimental but experimental codecs are not enabled, "
+               "add '-strict %d' if you want to use it.\n",
                codec_string, codec->name, FF_COMPLIANCE_EXPERIMENTAL);
         codec2 = av_codec_is_encoder(codec) ? avcodec_find_encoder(codec->id) : avcodec_find_decoder(codec->id);
         if (!(codec2->capabilities & CODEC_CAP_EXPERIMENTAL))
-            av_log(NULL, AV_LOG_ERROR, "Or use the non experimental %s '%s'.\n",
+            av_log(NULL, AV_LOG_ERROR, "Alternatively use the non experimental %s '%s'.\n",
                 codec_string, codec2->name);
         ret = AVERROR_EXPERIMENTAL;
         goto free_and_end;