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

an optional `+` work fine in html but not in pdf. changed to `{plus}` to make sure it renders plus sign in pdf

Author: kwhkim

book/10-git-internals/sections/refspec.asc

@@ -18,7 +18,7 @@ Running the command above adds a section to your repository's `.git/config` file
 	fetch = +refs/heads/*:refs/remotes/origin/*
 ----
 
-The format of the refspec is, first, an optional `+`, followed by `<src>:<dst>`, where `<src>` is the pattern for references on the remote side and `<dst>` is where those references will be tracked locally.
+The format of the refspec is, first, an optional `{plus}`, followed by `<src>:<dst>`, where `<src>` is the pattern for references on the remote side and `<dst>` is where those references will be tracked locally.
 The `+` tells Git to update the reference even if it isn't a fast-forward.
 
 In the default case that is automatically written by a `git remote add origin` command, Git fetches all the references under `refs/heads/` on the server and writes them to `refs/remotes/origin/` locally.