CTestCoverageCollectGCOV: Fix typo in ctest_coverage_collect_gcov docs

Docs added by commit f3e0b6f1eb (CTestCoverageCollectGCOV: Add module to
run gcov, 2015-01-14, v3.2.0-rc1~79^2) accidentally called the function
`cdash_coverage_collect_gcov`.  Rename `cdash` to `ctest`.

Modules/CTestCoverageCollectGCOV.cmake

@@ -20,7 +20,7 @@ This tarball also contains the following:
 After generating this tar file, it can be sent to CDash for display with the
 :command:`ctest_submit(CDASH_UPLOAD)` command.
 
-.. command:: cdash_coverage_collect_gcov
+.. command:: ctest_coverage_collect_gcov
 
   ::