Use lower-case commands in help

[varac]

Currently, /help otr shows commands in upper case:

…
TRUST [<fp>]
    Trust a specific fingerprint. The behavior is the same as the forget and
    distrust commands explained above.
…

But /otr TRUST results in OTR: Unknown command TRUST.
/otr trust will succeed.
Since the otr plugin is case-sensitive to commands, they should be documented in help the same way they are expected.