fix typo (#967)

zaptest/observer/observer.go

@@ -19,7 +19,7 @@
 // THE SOFTWARE.
 
 // Package observer provides a zapcore.Core that keeps an in-memory,
-// encoding-agnostic repesentation of log entries. It's useful for
+// encoding-agnostic representation of log entries. It's useful for
 // applications that want to unit test their log output without tying their
 // tests to a particular output encoding.
 package observer // import "go.uber.org/zap/zaptest/observer"