chore(gitignore): match nested target/ for all crates, not just the repo root

The repo has 7 Rust crates besides the root:

  affinescriptiser/, runtime/, distributions/rattlescript/,
  tools/affine-doc/, tools/affine-pkg/, tools/affinescript-dap/,
  tools/affinescript-lsp/

Each builds to its own sibling `target/` directory.  The previous rule
`/target/` was anchored to the repo root and only caught one of the
seven — `git status` was showing e.g. `tools/affinescript-lsp/target/`
as untracked across several sessions, one stray `git add .` away from
committing ~100 MB of build output.

Drop the leading slash so the rule matches `target/` anywhere — the
standard Rust convention.  No other glob change; the OCaml `/_build/`
remains anchored (we only have one `_build/`, at the root, and a
non-anchored `_build/` would over-match user code).

Verified post-fix that all three checked nested target/ dirs are now
ignored (`git check-ignore` returns the path).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: hyperpolymath

.gitignore

@@ -11,7 +11,7 @@ Thumbs.db
 .vscode/
 
 # Build
-/target/
+target/
 /_build/
 /_opam/
 /build/