[relax-ng] Some defects

While translating the RELAX NG spec, Kawamata-san has 
found some typos.

- "an externalRef atttribute" in 4.6 should read "an externalRef element"

- "an include atttribute" in 4.7 should read "an include element"

- "a choice" in 4.14 and 4.15 should read "a choice element" and 
  the word "choice" should be italicized.

Cheers,

Makoto