Fix typo in NaCl debugging docs

BUG=none
TEST="make" in native_client_sdk/src/doc

Review URL: https://codereview.chromium.org/69123003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@234291 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 234291