doc: fix three typos

doc/api/child_process.markdown

@@ -221,7 +221,7 @@ The `child.js` could look like this:
 
     process.on('message', function(m, socket) {
       if (m === 'socket') {
-        socket.end('You where handled as a ' + process.argv[2] + ' person');
+        socket.end('You were handled as a ' + process.argv[2] + ' person');
       }
     });
 

doc/api/cluster.markdown

@@ -69,7 +69,7 @@ This causes potentially surprising behavior in three edge cases:
    the worker to use the supplied handle, rather than talk to the master
    process.  If the worker already has the handle, then it's presumed
    that you know what you are doing.
-3. `server.listen(0)` Normally, this will case servers to listen on a
+3. `server.listen(0)` Normally, this will cause servers to listen on a
    random port.  However, in a cluster, each worker will receive the
    same "random" port each time they do `listen(0)`.  In essence, the
    port is random the first time, but predictable thereafter.  If you

doc/api/net.markdown

@@ -21,7 +21,7 @@ packet when the other end of the socket sends a FIN packet. The socket becomes
 non-readable, but still writable. You should call the `end()` method explicitly.
 See ['end'][] event for more information.
 
-Here is an example of a echo server which listens for connections
+Here is an example of an echo server which listens for connections
 on port 8124:
 
     var net = require('net');