Fix typo ("PASS" -> "FAIL")

[xtonyjiang]

Fixes #669

[devdinu]

func Test_Arguments_Diff_WithArgMatcher(t *testing.T) have the case to check assert with argumentMatcher and Diff is called for Assert, so when it's mismatched, output should've FAIL.

[bodnarbm]

@devdinu, I believe the test cited (Test_Arguments_Diff_WithArgMatcher) only asserts the portion of the message after the PASS or FAIL statements. Thus, it was not caught here where the message had the proper "not matched by" output, but had the erroneous "PASS".

[bodnarbm]

LGTM. We may wish to consider if "PASS" and "FAIL" should be included in the expected strings in Test_Arguments_Diff_WithArgMatcher

[georgelesica-wf]

Re-running CI, I'll merge once it finishes.