Doc fix in README: say "Kover Maven Plugin" where you mean it, not "Kover Gradle Plugin"

[binkley]

Thank you again for implementing #51!

Describe the bug
A trivial doc fix: the README.md says "Gradle" where it means to say "Maven".
See the line starting "For full information about latest stable release".

Kover Maven Plugin
The Kover Maven Plugin can be applied by specifying build plugin

<plugin>
    <groupId>org.jetbrains.kotlinx</groupId>
    <artifactId>kover-maven-plugin</artifactId>
    <version>0.8.3</version>
</plugin>
The list of Kover goals is specified in [this document section](https://kotlin.github.io/kotlinx-kover/maven-plugin#goals).

For full information about latest stable release of Kover Gradle Plugin, please refer to the [documentation](https://kotlin.github.io/kotlinx-kover/maven-plugin).

Errors
n/a

Expected behavior
README.md should say "Kover Maven Plugin" and not "Kover Gradle Plugin" in this section of the README.

Reproducer
n/a

Reports
n/a

Environment
n/a

[shanshin]

Thanks! :)