bpf, docs: Fix typos in instruction-set.rst

bpf, docs: Fix typos in instruction-set.rst

* "BPF ADD" should be "BPF_ADD".
* "src" should be "src_reg" in several places.  The latter is the field name
  in the instruction.  The former refers to the value of the register, or the
  immediate.
* Add '' around field names in one sentence, for consistency with the rest
  of the document.

Signed-off-by: Dave Thaler <dthaler1968@gmail.com>
Acked-by: David Vernet <void@manifault.com>
Link: https://lore.kernel.org/r/20240221173535.16601-1-dthaler1968@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>