reduce memory for plugin versions generation

follow-up after elastic/logstash#12763 is merged (on master)

have successfully tested  `./gradlew generatePluginsVersion` with 2g of heap, but let's not get overly optimistic 😉

Author: kares

build_docs.sh

@@ -33,7 +33,7 @@ git clone --depth 1 [email protected]:elastic/docs.git
 
 cd logstash
 
-./gradlew generatePluginsVersion -Dorg.gradle.jvmargs="-Xmx12g"
+./gradlew generatePluginsVersion -Dorg.gradle.jvmargs="-Xmx4g"
 
 cd ../docs-tools