Minor change to match the style guide.

Doc/ext/ext.tex

@@ -1439,7 +1439,7 @@ lock protecting Python's entire object space.  However, it is possible
 to temporarily release this lock using the macro
 \code{Py_BEGIN_ALLOW_THREADS}, and to re-acquire it using
 \code{Py_END_ALLOW_THREADS}.  This is common around blocking I/O
-calls, to let other threads use the CPU while waiting for the I/O to
+calls, to let other threads use the processor while waiting for the I/O to
 complete.  Obviously, the following function has the same problem as
 the previous one: