Update README to favor ERDDAP_MEMORY usage (same Xms/Xms)

[srstsavage]

In ERDDAP/erddap#185 @rmendels suggested that min and max memory should be set to the same value to prevent costly growing and shrinking of the heap.

While use cases may vary, in general this is probably a good idea, and users who want to set these to different values can still do so via explicit ERDDAP_MIN_MEMORY/ERDDAP_MAX_MEMORY.