[core] Fix for system memory output

[richardliaw]

Previously, even when allocating memory, the stderr would show:

Warning: Reducing object store memory because /dev/shm has only 65654784 bytes available. You may be able to free up space by deleting files in /dev/shm. If you are inside a Docker container, you may need to pass an argument with the flag '--shm-size' to 'docker run'.
Starting the Plasma object store with 0.00 GB memory.

when it should actually show

Starting the Plasma object store with 0.05 GB memory.

[AmplabJenkins]

Test PASSed.
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https://amplab.cs.berkeley.edu/jenkins//job/Ray-PRB/8557/
Test PASSed.