Actualise store descriptor property format for 2.3 docs

[dtaimanov]

According to https://github.com/jmix-framework/jmix/blob/master/jmix-core/core/src/main/java/io/jmix/core/impl/StoreDescriptorsRegistry.java#L95

(4) from https://docs.jmix.io/jmix/data-model/data-stores.html#custom should look like

jmix.core.store-descriptor-inmem = inMemoryStoreDescriptor

instead of

jmix.core.store-descriptor_inmem = inMemoryStoreDescriptor

And jmix.core.storeDescriptor-<store_name> instead of jmix.core.storeDescriptor_<store_name> in text above

Task:

1. Actualise text and code in 2.3
2. Check which versions need this fix too