doc: fix spelling errors

Neccessary -> Necessary
formated   -> formatted
thee       -> the
eventhough -> even though
seperately -> separately

Reviewed-by: Michael Niedermayer <michaelni@gmx.at>
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>

doc/examples/transcoding.c

@@ -117,7 +117,7 @@ static int open_output_file(const char *filename)
             /* in this example, we choose transcoding to same codec */
             encoder = avcodec_find_encoder(dec_ctx->codec_id);
             if (!encoder) {
-                av_log(NULL, AV_LOG_FATAL, "Neccessary encoder not found\n");
+                av_log(NULL, AV_LOG_FATAL, "Necessary encoder not found\n");
                 return AVERROR_INVALIDDATA;
             }