examples/muxing: fix typo: allocated -> allocate

doc/examples/demuxing.c

@@ -209,7 +209,7 @@ int main (int argc, char **argv)
     /* register all formats and codecs */
     av_register_all();
 
-    /* open input file, and allocated format context */
+    /* open input file, and allocate format context */
     if (avformat_open_input(&fmt_ctx, src_filename, NULL, NULL) < 0) {
         fprintf(stderr, "Could not open source file %s\n", src_filename);
         exit(1);