Clarify more wording when reading from stdin

Update the word 'initial' to be 'first', as this is more ubiquitous and
potentially easier for non-native speakers.

Clarify that it is the highlighting that may not work, and the input
will still be displayed regardless.

README.md

@@ -62,9 +62,9 @@ Display multiple files at once
 > bat src/*.rs
 ```
 
-Read from stdin, determine the syntax automatically (note, this will only work
-if the syntax can be determined from the initial line of the file, usually
-through a shebang such as `#!/bin/sh`)
+Read from stdin, determine the syntax automatically (note, highlighting will
+only work if the syntax can be determined from the first line of the file,
+usually through a shebang such as `#!/bin/sh`)
 
 ```bash
 > curl -s https://sh.rustup.rs | bat