Improve meek's description in the glossary

A translator pointed out (thanks @Himmel!):

"These [pluggable transports](#pluggable-transports) all make it look 
like you are browsing a major web site instead of using
[Tor](#tor-network-core-tor)."

Since Meek is one of the Pluggable Transports, I think this statement 
is not accurate as a description of Meek.
https://support.torproject.org/glossary/#meek

I agree and think we can rephrase this bit better.

cc @emmapeel