Fixes #745 correct spelling mistake

exercises/largest-series-product/src/example/java/LargestSeriesProductCalculator.java

         if (stringToSearch == null) {
             throw new IllegalArgumentException("String to search must be non-null.");
         } else if (!stringToSearch.chars().allMatch(Character::isDigit)) {
-            throw new IllegalArgumentException("String to search may only contains digits.");
+            throw new IllegalArgumentException("String to search may only contain digits.");
         }
     }
 

exercises/largest-series-product/src/test/java/LargestSeriesProductCalculatorTest.java

     @Test
     public void testStringToSearchContainingNonDigitCharacterIsRejected() {
         expectedException.expect(IllegalArgumentException.class);
-        expectedException.expectMessage("String to search may only contains digits.");
+        expectedException.expectMessage("String to search may only contain digits.");
 
         new LargestSeriesProductCalculator("1234a5");
     }