Minor readme fixes.

Author: dmitry-ivanov

README.md

@@ -240,7 +240,7 @@ $date = to_default_timezone('2017-02-28T14:05:01Z');
 
 #### `backtrace_as_string()`
 
-Get backtrace without arguments as a string:
+Get backtrace without arguments, as a string:
 
 ```php
 $backtrace = backtrace_as_string();