docs: Fix the EBNF for the '=' terminal symbol
docs: Fix the EBNF for the '=' terminal symbol

The rule-body non-terminal symbol did not quote the '=' symbol.

Signed-off-by: marco <3829486b93ec44395f0b980424bae9b6fb07b7bc@styra.com>

open-policy-agent/opa,open-policy-agent/opa,tsandall/opa,tsandall/opa,open-policy-agent/opa,tsandall/opa,open-policy-agent/opa,tsandall/opa,open-policy-agent/opa,open-policy-agent/opa,tsandall/opa,tsandall/opa