[lldb][docs] Fix plain text markers in Contributing
RST tries to resolve things in single backticks to a reference,
which is not the intention here. Double backticks indicates
plain text formatting.
Fixes warnings in the docs build:
contributing.rst:92: WARNING: 'any' reference target not found: A1
contributing.rst:92: WARNING: 'any' reference target not found: B1
contributing.rst:92: WARNING: 'any' reference target not found: B2
contributing.rst:92: WARNING: 'any' reference target not found: A2
contributing.rst:95: WARNING: 'any' reference target not found: A1->B1
contributing.rst:95: WARNING: 'any' reference target not found: B2->C2
contributing.rst:95: WARNING: 'any' reference target not found: C3->A3
contributing.rst:100: WARNING: 'any' reference target not found: LLDB_ACCEPTABLE_PLUGIN_DEPENDENCIES
contributing.rst:100: WARNING: 'any' reference target not found: LLDB_TOLERATED_PLUGIN_DEPENDENCIES
