zend_string docs: add missing apostrophe

[skip ci]

Author: DanielEScherzer

docs/source/core/data-structures/zend_string.rst

@@ -29,7 +29,7 @@ in bytes, and the ``val`` field contains the actual string data.
 
 You may wonder why the ``val`` field is declared as ``char val[1]``. This is called the `struct
 hack`_ in C. It is used to create structs with a flexible size, namely by allowing the last element
-to be expanded arbitrarily. In this case, the size of ``zend_string`` depends on the strings length,
+to be expanded arbitrarily. In this case, the size of ``zend_string`` depends on the string's length,
 which is determined at runtime (see ``_ZSTR_STRUCT_SIZE``). When allocating the string, we append
 enough bytes to the allocation to hold the strings content.