Bug 1270807 - fix grammar mistake in English version of badStsCertExplanation, r=flod

MozReview-Commit-ID: Eggesz6WlbM

--HG--
extra : rebase_source : 68868f82a534772336f3913e2ff55ee929cd1304

browser/locales/en-US/chrome/overrides/netError.dtd

@@ -196,7 +196,7 @@ was trying to connect. -->
 
 <!ENTITY certerror.pagetitle1  "Insecure Connection">
 <!ENTITY certerror.whatShouldIDo.badStsCertExplanation "This site uses HTTP
-Strict Transport Security (HSTS) to specify that &brandShortName; only connect
+Strict Transport Security (HSTS) to specify that &brandShortName; may only connect
 to it securely. As a result, it is not possible to add an exception for this
 certificate.">
 <!ENTITY certerror.copyToClipboard.label "Copy text to clipboard">