Fix doc.go error capitalization.

Change-Id: I98d31542563c15f8fa5a45db00f94d7a9aaa56bb
Reviewed-on: https://boringssl-review.googlesource.com/31964
Commit-Queue: David Benjamin <davidben@google.com>
Commit-Queue: Adam Langley <agl@google.com>
Reviewed-by: Adam Langley <agl@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>

util/doc.go

diff --git a/util/doc.go b/util/doc.go
index ab21547..33bcc66 100644
--- a/util/doc.go
+++ b/util/doc.go
@@ -453,7 +453,7 @@
 						ok = strings.HasPrefix(name, subject[:l-1])
 					}
 					if !ok {
-						return nil, fmt.Errorf("Comment for %q doesn't seem to match line %s:%d\n", name, path, declLineNo)
+						return nil, fmt.Errorf("comment for %q doesn't seem to match line %s:%d\n", name, path, declLineNo)
 					}
 				}
 				anchor := sanitizeAnchor(name)