Fix typo

Related to https://github.com/google/flatbuffers/pull/3904#issuecomment-224724181

php/FlatbufferBuilder.php

@@ -245,7 +245,7 @@ class FlatbufferBuilder
     public function putLong($x)
     {
         if ($x > PHP_INT_MAX) {
-            throw new \InvalidArgumentException("your platform can't handle long correctly. use 64bit machine.");
+            throw new \InvalidArgumentException("Your platform can't handle long correctly. Use a 64bit machine.");
         }
 
         $this->bb->putLong($this->space -= 8, $x);
@@ -257,7 +257,7 @@ class FlatbufferBuilder
     public function putUlong($x)
     {
         if ($x > PHP_INT_MAX) {
-            throw new \InvalidArgumentException("your platform can't handle ulong correctly. this is php limitations. please wait extension release.");
+            throw new \InvalidArgumentException("Your platform can't handle ulong correctly. This is a php limitation. Please wait for the extension release.");
         }
 
         $this->bb->putUlong($this->space -= 8, $x);