[Fix] missing + in pdf, 10-git-internals/refspec

[kwhkim]

• [x] I provide my work under the project license.
• [x] I grant such license of my work as is required for the purposes of future print editions to Ben Straub and Scott Chacon.

Changes

• changed + to {plus} in 10-git-internals/refspec.asc
  • "an optional + work" fine in html but not in pdf, resulted in changed to {plus} to make sure it renders plus sign in pdf

Context

This is quite self-obvious so I did not open the Issue