Help: Add comma to assist parsing of ctest_build command docs.

Help/command/ctest_build.rst

@@ -23,7 +23,7 @@ The APPEND option marks results for append to those previously
 submitted to a dashboard server since the last ctest_start.  Append
 semantics are defined by the dashboard server in use.
 
-If set the contents of the variable CTEST_BUILD_FLAGS are passed as
+If set, the contents of the variable CTEST_BUILD_FLAGS are passed as
 additional arguments to the underlying build command. This can e.g. be
 used to trigger a parallel build using the -j option of make. See
 :module:`ProcessorCount` for an example.