[2.7] bpo-33804: Document that the argument for io.TextIOBase.read() is...

[2.7] bpo-33804: Document that the argument for io.TextIOBase.read() is optional (GH-7510). (GH-8171)

(cherry picked from commit b6bb77c2)
Co-authored-by: Andrés Delfino <adelfino@gmail.com>

Doc/library/io.rst

@@ -724,7 +724,7 @@ Text I/O
 
       .. versionadded:: 2.7
 
-   .. method:: read(n)
+   .. method:: read(n=-1)
 
       Read and return at most *n* characters from the stream as a single
       :class:`unicode`.  If *n* is negative or ``None``, reads until EOF.