[SPARK-4656][Doc] Typo in Programming Guide markdown

Grammatical error in Programming Guide document

Author: lewuathe <lewuathe@me.com>

Closes #3412 from Lewuathe/typo-programming-guide and squashes the following commits:

a3e2f00 [lewuathe] Typo in Programming Guide markdown

(cherry picked from commit a217ec5fd5cd7addc69e538d6ec6dd64956cc8ed)
Signed-off-by: Josh Rosen <joshrosen@databricks.com>