doc: fix LaTex warning when building pdf docs

doc: fix LaTex warning when building pdf docs

LaTex doesn't like the Unicon character `≥` which should be represented
by the special sequnce `$\leq$`. Since this is buried all in Sphinx, and
we also have a scrip that look for the character, the easiest fix is to
just use `>=` instead which works without warnings, and without asking to
ignore every warning about the use of every instance of the special char.

Signed-off-by: Jafar Al-Gharaibeh <jafar@atcorp.com>