crypto/x509: revert change of article in SystemCertPool docs

The words 'the returned' were changed to 'a returned' in
8201b92 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: 6c83b80
GitHub-Pull-Request: #27438
Reviewed-on: https://go-review.googlesource.com/132776
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/crypto/x509/cert_pool.go

@@ -47,7 +47,7 @@ func (s *CertPool) copy() *CertPool {
 
 // SystemCertPool returns a copy of the system cert pool.
 //
-// Any mutations to a returned pool are not written to disk and do
+// Any mutations to the returned pool are not written to disk and do
 // not affect any other pool returned by SystemCertPool.
 //
 // New changes in the the system cert pool might not be reflected