Combat Knife DPS is less than the specific balancing point in GAME_BALANCE.md

### Describe the bug

Currently, the design document [GAME_BALANCE.md](https://github.com/CleverRaven/Cataclysm-DDA/blob/master/doc/GAME_BALANCE.md) says:
`Specific weapon balancing points: 20 - combat knives`
But the combat knife according to the actual DPS test (https://github.com/CleverRaven/Cataclysm-DDA/blob/master/tests/effective_dps_test.cpp) has:
`CHECK( calc_expected_dps( "knife_combat" ) == 19.0 );`

### Steps to reproduce

1. Read the linked design document
2. Read the linked tests
3. See the mismatch

### Expected behavior

I expect tests to match the design documents.

### Screenshots

_No response_

### Versions and configuration

Game version: cdda-linux-tiles-x64-2022-09-01-1632
Mod list: default

### Additional context

_No response_

Compare doc/GAME_BALANCE.md with tests/effective_dps_test.cpp, starting with the combat knife balancing point and its expected DPS assertion. Confirm which value the project intends to treat as authoritative, then update the relevant source and run the effective DPS test so the documented value and test expectation agree.

Written by the indexing model from the issue text.