docs: correct JVM options syntax for ELASTIC_MEMORY_SIZE

Documentation request

The documentation currently uses an uppercase G when specifying JVM heap settings (ELASTIC_MEMORY_SIZE) for the .env example at:

https://github.com/OpenCTI-Platform/opencti/blob/2457bc4063c045de48d43635881298673a2ad2e1/docs/docs/deployment/installation.md?plain=1#L156

The official OpenSearch and Elasticsearch documentation consistently uses lowercase g (for example, -Xms1g -Xmx1g).

While both formats are accepted by the JVM, aligning the documentation with the examples used in OpenSearch and Elasticsearch would improve consistency and reduce confusion for users.

References:

OpenSearch: https://docs.opensearch.org/latest/install-and-configure/install-opensearch/index/ Elasticsearch JVM settings: https://www.elastic.co/docs/reference/elasticsearch/jvm-settings

Additional context

No response