Use correct form of "Log in"

Use the verb form for correctness in context and consistency with text
on the h web pages.

src/sidebar/util/oauth-client.js

@@ -245,7 +245,7 @@ class OAuthClient {
       height: height,
     }).replace(/&/g, ',');
 
-    return $window.open('about:blank', 'Login to Hypothesis', authWindowSettings);
+    return $window.open('about:blank', 'Log in to Hypothesis', authWindowSettings);
   }
 }
 

src/sidebar/util/test/oauth-client-test.js

@@ -170,7 +170,7 @@ describe('sidebar.util.oauth-client', () => {
       assert.calledWith(
         fakeWindow.open,
         'about:blank',
-        'Login to Hypothesis',
+        'Log in to Hypothesis',
         'height=430,left=274.5,top=169,width=475'
       );
     });