Fix typo in the strings chapter (#24)

Author: stof

Book/php7/internal_types/strings.rst

@@ -7,7 +7,7 @@ Many operations are needed with strings. Although the C standard library provide
 C classical strings, aka ``char *`` (or ``char []``) are usually a little bit weak to use as-is in a strong program 
 like PHP is.
 
-Thus, PHP designed a layer on top of C strings: ``zend_strings``. Also, anoter API exists that implements common string 
+Thus, PHP designed a layer on top of C strings: ``zend_strings``. Also, another API exists that implements common string 
 operations both for C classical strings, or for ``zend_strings``: ``smart_str`` API.
 
 .. toctree::