Fix self-referential error_call default in section_topics

[sims1253]

section_topics() declared its error context as:

section_topics <- function(
  pkg,
  match_strings,
  error_path,
  error_call = error_call()
)

The default refers to the argument it initializes. If the function is called without an explicit error_call, evaluating the default fails with "promise already under evaluation: a default argument referenced itself".

The default is now caller_env(), which is the convention select_topics(), all_sign(), and match_eval() in the same file already use.

Found during my most recent ry audit.