doc: address nits for src/README.md

Refs: #30552 (review)

PR-URL: #30693
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Jiawen Geng <technicalcute@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>

Author: addaleax

src/README.md

@@ -795,7 +795,7 @@ heap if it is larger. This can be useful for performantly allocating temporary
 data if it is typically expected to be small (e.g. file paths).
 
 The `Utf8Value`, `TwoByteValue` (i.e. UTF-16 value) and `BufferValue`
-(`Utf8Value` but copy data from a `Buffer` is that is passed) helpers
+(`Utf8Value` but copy data from a `Buffer` if one is passed) helpers
 inherit from this class and allow accessing the characters in a JavaScript
 string this way.