core: Fix minor grammatical error (GoogleChrome#3638)

http://www.grammarbook.com/punctuation/semicolons.asp

_Noticed this during the ChromeDevSummit talk this morning._

lighthouse-core/audits/load-fast-enough-for-pwa.js

@@ -113,7 +113,7 @@ class LoadFastEnough4Pwa extends Audit {
           return {
             rawValue: true,
             // eslint-disable-next-line max-len
-            debugString: `First Interactive was found at ${Util.formatMilliseconds(timeToFirstInteractive)}, however, the network request latencies were not sufficiently realistic, so the performance measurements cannot be trusted.`,
+            debugString: `First Interactive was found at ${Util.formatMilliseconds(timeToFirstInteractive)}; however, the network request latencies were not sufficiently realistic, so the performance measurements cannot be trusted.`,
             extendedInfo,
             details,
           };