Update mg_url_decode.md

docs/c-api/http.h/mg_url_decode.md

@@ -7,12 +7,12 @@ signature: |
                     int is_form_url_encoded);
 ---
 
-Decode URL-encoded string.
+Decodes URL-encoded string.
 
 Source string is specified by (`src`, `src_len`), and destination is
 (`dst`, `dst_len`). If `is_form_url_encoded` is non-zero, then
 `+` character is decoded as a blank space character. This function
 guarantees to `\0`-terminate the destination. If destination is too small,
-then source string is partially decoded and `-1` is returned. Otherwise,
+then the source string is partially decoded and `-1` is returned. Otherwise,
 a length of decoded string is returned, not counting final `\0`.