Attribute not supported in HTML5 when run javadoc with JDK11

[rdnovell]

Hello, when I run "./gradlew javadoc --stacktrace" using JDK 11.0.2-open or 11.0.18-zulu I get:

[user@localhost ReactiveX-RxJava]$ ./gradlew javadoc --stacktrace

> Task :javadoc
/tmp/ReactiveX-RxJava/src/main/java/io/reactivex/rxjava3/observers/package-info.java:23: error: attribute not supported in HTML5: summary
 * <table border="1" style="border-collapse: collapse;" summary="The available observer types.">
                                                        ^
/tmp/ReactiveX-RxJava/src/main/java/io/reactivex/rxjava3/subjects/package-info.java:21: error: attribute not supported in HTML5: summary
 * <table border="1" style="border-collapse: collapse;" summary="The available subject classes with their respective base classes and consumer interfaces.">
                                                        ^
2 errors

Based on w3c doc
Note: In HTML5 the summary attribute is obsolete. Assistive technologies may or may not continue to support the attribute. Authors should consider alternatives and only use with caution.

I created a PR solving this situation.