Polish documentation

See gh-30498

spring-boot-project/spring-boot-docs/src/docs/asciidoc/features/developing-web-applications.adoc

@@ -628,7 +628,7 @@ The first step to customizing this feature often involves using the existing mec
 For that, you can add a bean of type `ErrorAttributes`.
 
 To change the error handling behavior, you can implement `ErrorWebExceptionHandler` and register a bean definition of that type.
-Because a `ErrorWebExceptionHandler` is quite low-level, Spring Boot also provides a convenient `AbstractErrorWebExceptionHandler` to let you handle errors in a WebFlux functional way, as shown in the following example:
+Because an `ErrorWebExceptionHandler` is quite low-level, Spring Boot also provides a convenient `AbstractErrorWebExceptionHandler` to let you handle errors in a WebFlux functional way, as shown in the following example:
 
 [source,java,indent=0,subs="verbatim"]
 ----

spring-boot-project/spring-boot-docs/src/docs/asciidoc/features/external-config.adoc

@@ -456,7 +456,7 @@ The use of placeholders with and without defaults is shown in the following exam
 	  description: "${app.name} is a Spring Boot application written by ${username:Unknown}"
 ----
 
-Assuming that the `username` property has not be set elsewhere, `app.description` will have the value `MyApp is a Spring Boot application written by Unknown`.
+Assuming that the `username` property has not been set elsewhere, `app.description` will have the value `MyApp is a Spring Boot application written by Unknown`.
 
 TIP: You can also use this technique to create "`short`" variants of existing Spring Boot properties.
 See the _<<howto#howto.properties-and-configuration.short-command-line-arguments>>_ how-to for details.