crypto/x509: revert change of article in SystemCertPool docs

The words 'the returned' were changed to 'a returned' in
8201b92a when referring to the value
returned by SystemCertPool. Brad Fitz pointed out after that commit was
merged that it makes the wording of this function doc inconsistent with
rest of the stdlib since 'a returned' is not used anywhere, but 'the
returned' is frequently used.

Fixes #27385

Change-Id: I289b533a5a0b5c63eaf0abb6dec0085388ecf76b
GitHub-Last-Rev: 6c83b8025704e291ebe5b15dd2ac3fa65b1b48ff
GitHub-Pull-Request: golang/go#27438
Reviewed-on: https://go-review.googlesource.com/132776


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>