typo in CHANGELOG.md

[juzhao]

https://github.com/prometheus/node_exporter/blob/master/CHANGELOG.md

[ENHANCMMENT] Reduce timer GC delays in the Linux filesystem collector #2169
[ENHANCMMENT] Add TCPTimeouts to netstat default filter #2189
[ENHANCMMENT] Use SysctlTimeval for boottime collector on BSD #2208

ENHANCMMENT should be ENHANCEMENT

The flag library has been changed, all flags now require double-dashs. (-foo becomes --foo)

double-dashs should be double-dashes

[juzhao]

PR: #2717