Add missing "%" format specifier to format strings

D'oh - still getting to grips with these format specifier macros!

Author: saxbophone

saxbospiral/render_backends/backend_pbm.c

@@ -66,8 +66,8 @@ sxbp_status_t sxbp_render_backend_pbm(
     // these are used to keep track of how many digits each is
     int width_string_length, height_string_length = 0;
     // convert width and height to a decimal string, store lengths
-    width_string_length = sprintf(width_string, PRIu64, bitmap.width);
-    height_string_length = sprintf(height_string, PRIu64, bitmap.height);
+    width_string_length = sprintf(width_string, "%" PRIu64, bitmap.width);
+    height_string_length = sprintf(height_string, "%" PRIu64, bitmap.height);
     /*
      * now that we know the length of the image dimension strings, we can now
      * calculate how much memory we'll have to allocate for the image buffer