Bug 1550422 - P3. Fix typo when filtering preferences to sync. r=njn

Differential Revision: https://phabricator.services.mozilla.com/D30588

--HG--
extra : moz-landing-system : lando

dom/ipc/ContentParent.cpp

@@ -2969,7 +2969,7 @@ ContentParent::Observe(nsISupports* aSubject, const char* aTopic,
         BLACKLIST_ENTRY(u"datareporting.policy."),
         BLACKLIST_ENTRY(u"browser.safebrowsing.provider."),
         BLACKLIST_ENTRY(u"browser.shell."),
-        BLACKLIST_ENTRY(u"browser.slowstartup."),
+        BLACKLIST_ENTRY(u"browser.slowStartup."),
         BLACKLIST_ENTRY(u"extensions.getAddons.cache."),
         BLACKLIST_ENTRY(u"media.gmp-manager."),
         BLACKLIST_ENTRY(u"media.gmp-gmpopenh264."),