Standardize spelling: occured --> occurred

Signed-off-by: Gabriel Rosenhouse <grosenhouse@pivotal.io>

internal/assertion/assertion_test.go

diff --git a/internal/assertion/assertion_test.go b/internal/assertion/assertion_test.go
index de9eff2..c03b7a3 100644
--- a/internal/assertion/assertion_test.go
+++ b/internal/assertion/assertion_test.go
@@ -241,7 +241,7 @@
 				e := recover()
 				RegisterFailHandler(Fail)
 				if e == nil {
-					Fail("expected a panic to have occured")
+					Fail("expected a panic to have occurred")
 				}
 			}()
 

internal/asyncassertion/async_assertion_test.go

diff --git a/internal/asyncassertion/async_assertion_test.go b/internal/asyncassertion/async_assertion_test.go
index 4288d4d..3d7e348 100644
--- a/internal/asyncassertion/async_assertion_test.go
+++ b/internal/asyncassertion/async_assertion_test.go
@@ -144,7 +144,7 @@
 					e := recover()
 					RegisterFailHandler(Fail)
 					if e == nil {
-						Fail("expected a panic to have occured")
+						Fail("expected a panic to have occurred")
 					}
 				}()
 
@@ -278,7 +278,7 @@
 					e := recover()
 					RegisterFailHandler(Fail)
 					if e == nil {
-						Fail("expected a panic to have occured")
+						Fail("expected a panic to have occurred")
 					}
 				}()
 

matchers/have_occurred_matcher.go

diff --git a/matchers/have_occurred_matcher.go b/matchers/have_occurred_matcher.go
index 3dfe9e1..c96dfee 100644
--- a/matchers/have_occurred_matcher.go
+++ b/matchers/have_occurred_matcher.go
@@ -21,7 +21,7 @@
 }
 
 func (matcher *HaveOccurredMatcher) FailureMessage(actual interface{}) (message string) {
-	return fmt.Sprintf("Expected an error to have occured.  Got:\n%s", format.Object(actual, 1))
+	return fmt.Sprintf("Expected an error to have occurred.  Got:\n%s", format.Object(actual, 1))
 }
 
 func (matcher *HaveOccurredMatcher) NegatedFailureMessage(actual interface{}) (message string) {