docs: fix simple typo, specifing -> specifying (#614)
There is a small typo in pb_encode.h, spm_headers/nanopb/pb_encode.h.
Should read `specifying` rather than `specifing`.
diff --git a/pb_encode.h b/pb_encode.h
index 88e246a..9cff22a 100644
--- a/pb_encode.h
+++ b/pb_encode.h
@@ -132,7 +132,7 @@
* structure. Call this from the callback before writing out field contents. */
bool pb_encode_tag_for_field(pb_ostream_t *stream, const pb_field_iter_t *field);
-/* Encode field header by manually specifing wire type. You need to use this
+/* Encode field header by manually specifying wire type. You need to use this
* if you want to write out packed arrays from a callback field. */
bool pb_encode_tag(pb_ostream_t *stream, pb_wire_type_t wiretype, uint32_t field_number);
