cleanup: Clarify CaptivePortal routine problems

Make some of the problem descriptions more succint.

TEST=unit_tests --gtest_filter=CaptivePortalRoutineTest.*
BUG=chromium:956783

Change-Id: Ie3b666e0aa999a4d387ef0ed4634364df18966dd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2490176
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Commit-Queue: Kartik Hegde <khegde@chromium.org>
Cr-Commit-Position: refs/heads/master@{#820354}

chrome/browser/chromeos/net/network_diagnostics/README.md

@@ -127,10 +127,9 @@ Tests whether the internet connection is behind a captive portal.
 
 Problems:
 * `kNoActiveNetworks`: No active networks found.
-* `kRestrictedConnectivity`: The active network is detected to be behind a
-   captive portal and have restricted connectivity.
-* `kCaptivePortalState`: The active network is detected to be behind a captive
-   portal.
+* `kRestrictedConnectivity`: The active network is behind a captive portal and
+   has restricted connectivity.
+* `kCaptivePortalState`: The active network is behind a captive portal.
 
 ### Firewall Routines