docs: fix typo in faq.rst (#1619)

CTrace --> CTracer

Author: charleswhchan

doc/faq.rst

@@ -121,7 +121,7 @@ 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
-``CTrace: available`` or ``CTracer: unavailable``.  If it says unavailable,
+``CTracer: available`` or ``CTracer: unavailable``.  If it says unavailable,
 then you are using the slow Python implementation.
 
 Try re-installing coverage.py to see what happened and if you get the CTracer