docs: fix references in migration guide

Author: ocpalo

docs/migrate-libpng.md

@@ -255,8 +255,8 @@ See also: [Progressive image decoding](decode.md#progressive-image-decoding)
 
 | libpng              | spng                                                   | Notes                               |
 |---------------------|--------------------------------------------------------|-------------------------------------|
-| `png_read_row()`    | [`spng_decode_row()`](encode.md#spng_decode_row)       |                                     |
-| `png_write_image()` | [`spng_encode_image()`](encode.md#spng_decode_image)   |                                     |
+| `png_read_row()`    | [`spng_decode_row()`](decode.md#spng_decode_row)       |                                     |
+| `png_read_image()`  | [`spng_decode_image()`](decode.md#spng_decode_image)   |                                     |
 | `png_read_info()`   | [`spng_decode_chunks()`](decode.md#spng_decode_chunks) | Optional, chunks are read on-demand |
 | `png_read_end()`    | [`spng_decode_chunks()`](decode.md#spng_decode_chunks) | Optional, chunks are read on-demand |
 
@@ -388,4 +388,4 @@ Filter choice flags for [`spng_set_option()`](context.md#spng_set_option) when u
 | `png_write_chunk()`                 | N/A                                         | Not supported                                     |
 | `png_write_chunk_start()`           | N/A                                         | Not supported                                     |
 | `png_write_chunk_data()`            | N/A                                         | Not supported                                     |
-| `png_write_chunk_end()`             | N/A                                         | Not supported                                     |
+| `png_write_chunk_end()`             | N/A                                         | Not supported                                     |