Fix minor javadoc error

[JonathanGiles]

Fix minor javadoc error preventing javadocs from being successfully generated (even though it is internal API)

[moderakh]

@JonathanGiles thanks for the PR.

I wonder how you are trying to generate the javadocs? as we have the following maven configuration for maven-javadoc-plugin it shouldn't event attempt to generate any javadoc for RxCollectionCache.java. So when using maven javadoc gets generated without any error.

<plugin>
        <groupId>org.apache.maven.plugins</groupId>
        <artifactId>maven-javadoc-plugin</artifactId>
        <version>2.9.1</version>
        <configuration>
          <links>
            <link>http://reactivex.io/RxJava/javadoc/</link>
          </links>
          <sourceFileExcludes>
            <sourceFileExclude>**/internal/**/*.java</sourceFileExclude>
            <sourceFileExclude>**/*Internal.java</sourceFileExclude>
          </sourceFileExcludes>
        </configuration>
        <executions>
          <execution>
            <id>attach-javadocs</id>
            <goals>
              <goal>jar</goal>
            </goals>
          </execution>
        </executions>
      </plugin>

[JonathanGiles]

I am running a build at https://github.com/JonathanGiles/azure-javadocs that runs its own maven pom to aggregate all Java SDK output here: https://jonathangiles.github.io/azure-javadocs/ This helps to identify compilation issues under JDK 9, and over time I have submitted a number of minor PRs to fix issues in JavaDocs.

Even though the above PR is in a non-public API, because it is specifying invalid JavaDoc, it is good to fix it.

[moderakh]

@JonathanGiles thanks for the explanation. Makes sense.