Update index.php

Change “Locator” for “Gridsquare” to standardise on one term throughout the UI.

application/views/gridsquares/index.php

@@ -56,7 +56,7 @@
 		      <th scope="col">Callsign</th>
 		      <th scope="col">Mode</th>
 		      <th scope="col">Band</th>
-		      <th scope="col">Locator</th>
+		      <th scope="col">Gridsquare</th>
 		    </tr>
 		  </thead>
 		  <tbody>