docs: correct the description of 'debug sys' tracer details. coveragepy#1157

Author: nedbat

doc/faq.rst

@@ -90,8 +90,8 @@ Make sure you are using the C trace function.  Coverage.py provides two
 implementations of the trace function.  The C implementation runs much faster.
 To see what you are running, use ``coverage debug sys``.  The output contains
 details of the environment, including a line that says either
-``tracer: CTracer`` or ``tracer: PyTracer``.  If it says ``PyTracer`` then you
-are using the slow Python implementation.
+``CTrace: available`` or ``CTracer: unavailable``.  If it says unavailble,
+then you are using the slow Python implementation.
 
 Try re-installing coverage.py to see what happened and if you get the CTracer
 as you should.