doc: fix grammar and wording in tls and timers

In `tls.markdown`, there was a misuse of 'a' which has been replaced
with 'an'.

In `timers.markdown`...
  line 31: misuse of 'a', replaced with 'an'
  line 59: unclear wording, haywire 'a', added new comma

Author: brendanashworth

doc/api/timers.markdown

@@ -28,7 +28,7 @@ you can also pass arguments to the callback.
 
 ## clearInterval(intervalObject)
 
-Stops a interval from triggering.
+Stops an interval from triggering.
 
 ## unref()
 
@@ -56,7 +56,7 @@ can also pass arguments to the callback.
 
 Callbacks for immediates are queued in the order in which they were created.
 The entire callback queue is processed every event loop iteration. If you queue
-an immediate from a inside an executing callback that immediate won't fire
+an immediate from inside an executing callback, that immediate won't fire
 until the next event loop iteration.
 
 ## clearImmediate(immediateObject)

doc/api/tls.markdown

@@ -221,7 +221,7 @@ automatically set as a listener for the [secureConnection][] event.  The
 
     NOTE: Automatically shared between `cluster` module workers.
 
-  - `sessionIdContext`: A string containing a opaque identifier for session
+  - `sessionIdContext`: A string containing an opaque identifier for session
     resumption. If `requestCert` is `true`, the default is MD5 hash value
     generated from command-line. Otherwise, the default is not provided.
 
@@ -501,7 +501,7 @@ connections using TLS or SSL.
 `function (tlsSocket) {}`
 
 This event is emitted after a new connection has been successfully
-handshaked. The argument is a instance of [tls.TLSSocket][]. It has all the
+handshaked. The argument is an instance of [tls.TLSSocket][]. It has all the
 common stream methods and events.
 
 `socket.authorized` is a boolean value which indicates if the