Docs: columnsForPixelGroups() is a function, and its default includes B

[eliotmcintire]

Documentation only — one metadata string, no code path changes.

     expectsInput("columnsForPixelGroups", "character",
                  paste("The names of the columns in `cohortData` that define unique `pixelGroup`s.",
-                       "Default is `c('ecoregionGroup', 'speciesCode', 'age')`;",
-                       "see `?LandR::columnsForPixelGroups`).")),
+                       "Default is `c('ecoregionGroup', 'speciesCode', 'age', 'B')`;",
+                       "see `?LandR::columnsForPixelGroups()`).")),

Two corrections

1. Parens. LandR changed columnsForPixelGroups from a character vector to a function (R/cohorts.R:1741). The bare form in this docstring now points a reader at the function object rather than the thing they want. The module's own code at line 1731 already calls it correctly with parens — this just brings the prose in line.

2. The stated default was wrong, independent of the above. It claimed c('ecoregionGroup', 'speciesCode', 'age'), but LandR returns four columns:

columnsForPixelGroups <- function() {
  c("ecoregionGroup", "speciesCode", "age", "B")
}

"B" was missing. I noticed this while fixing the parens and corrected it in the same string rather than leave a known-false default sitting next to a line I was already touching. Happy to split it out if you would rather keep the changes separate.

Scope

Companion to PredictiveEcology/Biomass_core#108, which fixes the one genuine code site (Biomass_core.R:870 assigned the function object instead of calling it). I audited both modules across all branches plus the rest of the organisation; this docstring was the only remaining bare reference anywhere, and being prose it was harmless — hence docs-only here.

File parses clean.