Fix incorrectly named npmSymbol

[dmwyatt]

I'm not sure this is the right way to fix this.

If you enable plc_npm_npmSymbol in the powerline config, then you get a crash because npmSymbol isn't actually defined anywhere. I think that was supposed to point to plc_npm_npmSymbol instead.

Of course, the config and this file hasn't been changed in many months so either no one is using this, no one has bothered to submit a fix, or I'm doing this all wrong.

[dmwyatt]

I see now that this was also fixed in the unrelated PR #53