📝 update documentation (#4723)

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

docs/mkdocs/docs/api/basic_json/is_binary.md

@@ -4,7 +4,7 @@
 constexpr bool is_binary() const noexcept;
 ```
 
-This function returns `#!cpp true` if and only if the JSON value is binary array.
+This function returns `#!cpp true` if and only if the JSON value is a binary array.
     
 ## Return value