style: environment variable names should be monospace

Author: nedbat

doc/changes.rst

@@ -547,7 +547,7 @@ Version 5.0a2 — 2018-09-03
     may need ``parallel=true`` where you didn't before.
 
   - The old data format is still available (for now) by setting the environment
-    variable COVERAGE_STORAGE=json. Please tell me if you think you need to
+    variable ``COVERAGE_STORAGE=json``. Please tell me if you think you need to
     keep the JSON format.
 
   - The database schema is guaranteed to change in the future, to support new
@@ -1521,7 +1521,7 @@ Version 4.0a6 — 2015-06-21
   persisted in pursuing this despite Ned's pessimism.  Fixes `issue 308`_ and
   `issue 324`_.
 
-- The COVERAGE_DEBUG environment variable can be used to set the
+- The ``COVERAGE_DEBUG`` environment variable can be used to set the
   ``[run] debug`` configuration option to control what internal operations are
   logged.
 

doc/cmd.rst

@@ -315,8 +315,8 @@ Data file
 .........
 
 Coverage.py collects execution data in a file called ".coverage".  If need be,
-you can set a new file name with the COVERAGE_FILE environment variable.  This
-can include a path to another directory.
+you can set a new file name with the ``COVERAGE_FILE`` environment variable.
+This can include a path to another directory.
 
 By default, each run of your program starts with an empty data set. If you need
 to run your program multiple times to get complete data (for example, because