Fix typos in Javadoc of assertable application context classes

Closes gh-10865

Author: wilkinsona

spring-boot-project/spring-boot-test/src/main/java/org/springframework/boot/test/context/assertj/ApplicationContextAssertProvider.java

@@ -27,7 +27,7 @@
 
 /**
  * An {@link ApplicationContext} that additionally supports AssertJ style assertions. Can
- * be used to decorate and existing application context or an application context that
+ * be used to decorate an existing application context or an application context that
  * failed to start.
  * <p>
  * Assertions can be applied using the standard AssertJ {@code assertThat(...)} style (see

spring-boot-project/spring-boot-test/src/main/java/org/springframework/boot/test/context/assertj/AssertableApplicationContext.java

@@ -24,7 +24,7 @@
 
 /**
  * An {@link ApplicationContext} that additionally supports AssertJ style assertions. Can
- * be used to decorate and existing application context or an application context that
+ * be used to decorate an existing application context or an application context that
  * failed to start.
  * <p>
  * See {@link ApplicationContextAssertProvider} for more details.

spring-boot-project/spring-boot-test/src/main/java/org/springframework/boot/test/context/assertj/AssertableReactiveWebApplicationContext.java

@@ -23,7 +23,7 @@
 
 /**
  * A {@link ReactiveWebApplicationContext} that additionally supports AssertJ style
- * assertions. Can be used to decorate and existing reactive web application context or an
+ * assertions. Can be used to decorate an existing reactive web application context or an
  * application context that failed to start.
  * <p>
  * See {@link ApplicationContextAssertProvider} for more details.

spring-boot-project/spring-boot-test/src/main/java/org/springframework/boot/test/context/assertj/AssertableWebApplicationContext.java

@@ -24,7 +24,7 @@
 
 /**
  * A {@link WebApplicationContext} that additionally supports AssertJ style assertions.
- * Can be used to decorate and existing servlet web application context or an application
+ * Can be used to decorate an existing servlet web application context or an application
  * context that failed to start.
  * <p>
  * See {@link ApplicationContextAssertProvider} for more details.