docs(readme): update command syntax

README.md

@@ -100,7 +100,7 @@ Available options:
 > a terminal of dimensions not smaller than the one used for recording, as
 > there's no "transcoding" of control sequences for new terminal size.
 
-### `export`
+### `export <filename>`
 
 **Export recorded asciicast to svg.**
 
@@ -109,12 +109,11 @@ This command exports given asciicast (as recorded by `rec` command) to svg.
 Exporting from a local file:
 
 ```sh
-termsvg export -i /path/to/asciicast.cast
+termsvg export /path/to/asciicast.cast
 ```
 
 Available options:
 
-- `-i, --input=<file>` - Asciicast file to export.
 - `-o, --output=<file>` - Output svg to be created. Defaults to [input].svg
 - `-m, --minify` - Minify svg using [Minify](https://github.com/tdewolff/minify)