fix README.md example

[Ynewtime]

The second example in README.md has minor typo which may cause confusion:

mvn jetty:run -Djetty.http.port=9999

notice there is no space between the -D option and the value, shall be updatd with:

mvn jetty:run -D jetty.http.port=9999

[HeinrichAD]

The missing space was intended.
See: https://books.sonatype.com/mvnref-book/reference/running-sect-options.html#running-sect-define-prop.

But I guess both ways seem to work just fine, as we use both conventions within the workflows:

plantuml-server/.github/workflows/tests.yml

Line 26 in 0552ada

run: mvn --batch-mode -f pom.jdk8.xml -D java.net.useSystemProxies=true -Dapache-jsp.scope=compile clean package

Since it's more readable with a space, I wouldn't mind changing it.
But I would prefer if we had only one convention within the README.
So, please also change the other 4 occurrences:

plantuml-server/README.md

Line 116 in 0552ada

mvn jetty:run -D THE_ENV_VARIABLE=THE_ENV_VALUE -Djetty.http.port=9999

plantuml-server/README.md

Line 138 in 0552ada

* Set PlantUML system properties (like over the Java command line using the `-Dpropertyname=value` syntax).

plantuml-server/README.md

Line 183 in 0552ada

mvn package -Dapache-jsp.scope=compile

plantuml-server/README.md

Line 189 in 0552ada

mvn package -f pom.jdk8.xml [-Dapache-jsp.scope=compile]