dd_C doesn't work in Complexes

I had mentioned this bug about the cohomological notation for getting the differentials of a complex in an email, but mentioning it again here. cc @ggsmith
```m2
i1 : R = kk[x,y]

o1 = R

o1 : PolynomialRing

i2 : C = koszulComplex vars R

1 2 1
o2 = R <-- R <-- R

0 1 2

o2 : Complex
```
`dd^C` works fine:
```m2
i3 : dd^C

1 2
o3 = 0 : R <----------- R : 1
| x y |

2 1
1 : R <-------------- R : 2
{1} | -y |
{1} | x |

o3 : ComplexMap
```
But `dd_C`, which is explicitly mentioned in the [documentation](https://macaulay2.com/doc/Macaulay2/share/doc/Macaulay2/Complexes/html/_differential_spof_spa_spchain_spcomplex.html), does not work:
```m2
i4 : dd_C

o4 = dd
1 2 1
R <-- R <-- R

0 1 2

o4 : IndexedVariable
```

Run the reported Macaulay2 session with a Koszul complex, comparing dd^C with dd_C. Read the linked differential documentation and trace the dd_C entry point; done means dd_C resolves to the complex differential and returns the expected ComplexMap rather than an IndexedVariable.

Written by the indexing model from the issue text.