(contributing.md) grammatical changes for clarity

Changed some wording for better grammatical structure.

BEFORE: "This is why we can't accept all submissions."
AFTER: "This is why we can't accept every submission."

BEFORE: "However, we're always open to look at novel ways to solve existing problems."
AFTER: "However, we're always open to looking at novel ways to solve existing problems."
Merge pull request #1092 from alextsg/patch-1

(contributing.md) grammatical changes for clarity