Fix minor grammar problem
Fix minor grammar problem

"a alpha" → "an alpha"
espdev/readthedocs.org,davidfischer/readthedocs.org,safwanrahman/readthedocs.org,tddv/readthedocs.org,espdev/readthedocs.org,davidfischer/readthedocs.org,tddv/readthedocs.org,pombredanne/readthedocs.org,espdev/readthedocs.org,safwanrahman/readthedocs.org,davidfischer/readthedocs.org,safwanrahman/readthedocs.org,rtfd/readthedocs.org,rtfd/readthedocs.org,safwanrahman/readthedocs.org,pombredanne/readthedocs.org,rtfd/readthedocs.org,davidfischer/readthedocs.org,pombredanne/readthedocs.org,tddv/readthedocs.org,rtfd/readthedocs.org,espdev/readthedocs.org,espdev/readthedocs.org