bpf, docs: Fix the description of 'src' in ALU instructions

An ALU instruction's source operand can be the value in the source
register or the 32-bit immediate value encoded in the instruction. This
is controlled by the 's' bit of the 'opcode'.

The current description explicitly uses the phrase 'value of the source
register' when defining the meaning of 'src'.

Change the description to use 'source operand' in place of 'value of the
source register'.

Signed-off-by: Puranjay Mohan <puranjay@kernel.org>
Acked-by: Dave Thaler <dthaler1968@gmail.com>
Link: https://lore.kernel.org/r/20240514130303.113607-1-puranjay@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>