A number of general dictionaries list erlang, with a lower case e, including Collins, Dictionary.com and Wiktionary. It looks like some users of the word capitalize it, while others don't. It is named after a person, the Danish mathematician and engineer A.K. Erlang, but it seems to be normal for units of measure to be written without capitals even when they're named after someone, e.g. newton, pascal. It can be found in both forms on the website of the International Telecommunications Union.
I'll add erlang as a rare word.