Update typos in old release notes

Two releases in 2012 were labeled as 2010.

release_notes.txt

@@ -1042,12 +1042,12 @@ Sep 17, 2012: libphonenumber-5.1.3
  - Updated metadata for non-geographical calling codes: 800, 808, 888, 979
  - Updated geocoding data for country calling code(s): 98 (en), 1 (en)
 
-Sep 11, 2010: libphonenumber-5.1.2
+Sep 11, 2012: libphonenumber-5.1.2
 * Bug fix:
  - Fixing regression in AsYouTypeFormatter where it no longer worked for numbers entered in national
    format for countries with no national prefix, e.g. Spain.
 
-Sep 5, 2010: libphonenumber-5.1.1
+Sep 5, 2012: libphonenumber-5.1.1
 * Code changes:
   - Added better logging/exception handling for catching cases where metadata is invalid/missing.